This is a very good thread. I agree with nearly everything, and while it's very generalist advice, I think it would be neat to provide specific examples of things that I ran into this past weekend as it may shed some light. Please excuse me if this is a long post.
I had a table at the St. Louis show this past weekend, and two events are outstanding in my mind that people think about and vendors sometimes run into.
1. There was a guy that one of the carvers I represent told me to use caution with. He knew this particular gentleman would be stopping by my table, was a fan of his pipes, and has a history of making really low offers. Okay, I says. Well, he did stop by my table and was looking at the pipes I was offering. I commented on the pipe he was smoking and we had a pleasant chat. He asked me if I could do any better on a $345 pipe, and to really consider it. I asked him to come back in 10 minutes and I would present him with my best offer. I work on consignment, and can't do much, but I'm willing to play ball. He came back, I gave him my offer and my reasonings (which I thought were valid), and he seemed to take it well and said he would consider it. Cool! After 20 minutes or so he returned to my table and offered me $275 for the same pipe. That's $70 dollars off a pipe from a maker that he already collects and loves, and he low-balls me. No, but thank you for doubling my offer with your counter-offer. I now know this man as a man who has good taste, a very tight wallet, and shrewd business sense. I'll remember him forever now, and know to proceed with caution. I wasn't offended by his counter-offer, but just couldn't go that low.
TL;DR: If you make an offer, don't low-ball. Vendors typically have long memories.
2. The second scenario was a pretty cool one. I had a pipe from a new maker on my table, and the man who had the table next to me was drooling on this pipe for the whole show. He asked me if I'd consider a lower price for it, 'sure', I says. He made an offer that was totally reasonable. He had some Lee Von Erck estates on his table that would make a neat offering on my website so I asked if he wanted to trade. Of course the LVE pipe was a higher dollar amount than the pipe I had, and he agreed to the offer if I would also pay the difference in the price; we danced for a few minutes and ended up trading pipes and I paid him $40 for the LVE estate I was interested in. We both walked away happy men.
TL;DR: For some people it's okay to ask if a trade would be worth considering. Personally, it doesn't bother me. Steve nailed it when he mentioned not to haggle with the pros. Just be prepared for a counter offer of some kind. Whether the guy on the other side of the table is doing it for fun or to try and get rich, just remember that in either scenario it's nice for them to come away from a transaction with a little meat left on the bone. Use your best judgement!
To add to what the others have mentioned, please be considerate of where you are in space/time. Quite a few guys I know who are vendors love a good chat.. myself included! But certainly NOT when people are trying to see what's on your table. Please be courteous of where you are standing!! If you want to chat, and it seems like the vendor is into it-- don't stand front and center at his table. Kindly move to the far left or right side of the table so that if someone wants to buy something, they don't have to look around you and your messenger bag, backpack, briefcase, luggage, or donkey laden with packages to do so. You'll know when you are standing incorrectly when you see the anxious 'hunted' look in the vendors eyes and a blanched skin tone. My other request is not to strike up a long conversation or continue a conversation when there is a crowd around that vendor's table. There is always time later to catch up and shoot the breeze; more than likely the vendor will be happy to chat.
It also never hurts to get familiar with a vendor you like. Personally, I'd be much more inclined to cut someone a better deal (looked or unlooked for) with an old friend than someone I've never seen before. Vendors are fond of old friends, repeat customers, and people who offer to buy them drinks. Our people take care of us, and we take care of our people.
The last thing that comes to mind specifically from this past weekend is a sad and scary one. Just an hour into the show, word traveled quickly that someone had STOLEN A PIPE; a pipe shape that this specific carver had only ever made one of. When word reached me, I was furious, concerned, and afraid in spades. Don't be the guy who steals pipes. I went from feeling like I was in a safe environment to an unsafe one in the space of a second. At shows, it's great for me to be able to leave my table and wander for short periods of time. I get to see what's available, catch up with old friends, and stretch my legs. After I received that news, I didn't feel comfortable leaving my table for any period of time. Luckily I traveled with a friend, and he was able to safeguard my table for my infrequent excursions.
---I'm going to get real dark for a second here---
I've heard stories of prison, and there are three kinds of inmates who have it the worst: Child molesters/murderers, women murderers, and interestingly enough, cop-murderers. Why cop-murderers you ask? Well, do you think the guards are going to stop the rape of a man who killed a cop? Nope. Heads get turned and that offender has the worst ten to fifteen years of his life. To bring things full-circle, if this thief were caught at the show... he would have been disemboweled and torn limb from limb. Not only would nobody have stopped anyone from causing him personal harm, but I'm sure there would have been a long cue to get in on the action. Don't be the guy who steals pipes. It's not the price that is the issue, it's the action.
---Sorry about that, I'll get back on track, here---
It's also a good thing to leave a table how you found it. For vendors, a successful show works in two parts: one part is offering things that people want to buy, the other part is presentation. I work awfully hard to make sure that my table is going to be presentable. Since my stock is constantly rotating, I make a mock table before each show to play around with layouts so my set-up time is minimal and efficient for the duration of the show. If you pick something up (something I strongly encourage), try and make a good effort of getting it back where it was. I'd much rather spend time with customers than fiddling around with my table trying to get things back where I want them to be. Don't be afraid to ask for a pipe cleaner as well. Vendors are usually prepared for this and I was pleasantly surprised I was asked for cleaners at the show for the 'pipe cleaner test'. It speaks well for the buyer.
The last thing I'll leave you with is an agreement with Wayne and Steve on their opinions. I think Wayne hit it pretty well when he mentioned the bit about the last few hours of a show idea. If someone were to approach me with a reasonable offer for a discount on a pipe in those last hours, I'd certainly be more inclined to do so. It's a good feeling to come home with a reduced inventory. It would make me feel better if I saw you a few times during the show and offered me something, because that shows at least you were honest about it. The stranger who shows up making an offer at the very end sort of comes off as a cheapskate. Maybe I'm wrong in that opinion, and in most cases there many have been circumstances involved in his/her tardiness, but that's just how it strikes me.
Pipe shows are fun, and I'd encourage everyone to try and make a few of 'em if they can. Nothing beats the selection and camaraderie of pipe shows. Cheers!