I'm late to the discussion, but I'll add that I've bought probably a dozen or more estate tins on eBay and have never had any problems.
Know what you're buying before you bid. I'm seeing a lot of listings these days for "vintage" tins that are "no longer available in stores" that clearly aren't vintage and clearly are available in stores, and almost always at lower prices. Some sellers seem to believe that if a tin is dated last year, it is "no longer in production." And some seem to believe that if it's currently out of stock, then it's "out of production." I've seen undated tins of Penzance being offered like this. (Or maybe they're just playing games with eBay restrictions; I don't know.) But especially if you're bidding on a tin from Rattray's, Dunhill, Balkan Sobranie -- or anything else that has been manufactured by multiple blenders or reissued in any way -- you need to know what you're looking at. Because most sellers are either completely ignorant about specifics or coy about them, hoping that you'll bid $200 for a 2oz. tin of the new Balkan Sobranie.
Case in point: I just did a "sealed tin" search on eBay and found one seller who has brand-new tins of Orlik's version of Dunhill's Nightcap and Early Morning Pipe. . . priced at just over $35.00 each! I don't get it. At the same time, I saw a truly rare tin of Sullivan Powell's Gentleman's Mixture which still has days to go but is already over $100, which actually seems fair. (It's a tall 50g canister rather than the round flat tin, and I've never seen one before in my life for Gentleman's Mixture, which is an outstanding blend in its own right.)
If the seller doesn't provide enough photos for you to see what you need to see in order to get at least a general idea of the age, then ask for additional photos. Some sellers are also in the habit of posting no information whatsoever about a tin's age. Unless a tin is clearly older, ask. (Just don't be surprised if they're actually trying to sell brand-new tins for vintage prices.)
This sounds like a no-brainer, but read any descriptions carefully and look at all of the posted photos. It's not unusual to see tins that appear to be sealed in the photos, while the description includes a note that the tin has been opened and there is no tobacco in it.
Finally, there are bargains to be found if you look carefully or if you're searching for something a little off the beaten path. I just sold two 6-year-old tins of a Virginia tobacco that went for about $2 more per-tin than I originally paid. I think that was a steal, given that the blend has six years of age on it. (And this auction got just a single bid; it went for my base asking price, even though there were multiple "watchers.") I've also seen a wide range of winning bids for the same blend over the course of a month; I've seen one of Pease's discontinued Syrians go for $35 one week and $60 the next. It just depends on who happens to be looking for it at any given time. So if you see an auction surpass your maximum, be patient; unless the tin is extremely rare, it's going to come up for sale again.
Bob