I've culled pipes from my collection a few times but I have maybe 10 or 15 pipes that I don't reach for that I won't be selling anytime soon. Some of them aren't worth that much in my opinion (about $20) but they have a history to them. I have a Salmon & Gluckstein straight billard that is over 100 years old, for example. I also have a couple of pipes stamped with the names of pipe shops that no longer exists. One is a duTreil, a pipe shop in New Orleans that went out of business in the late 1960s. The duTreil is also stamped "Algerian Briar Made in France."
On the other hand, I have sold a Dunhill Redbark, a Preben Holm Freehand and a Ben Wade by Preben Holm.
As for the original question, I will usually sell a pipe or two when I find a new pipe that I decide I want to buy.