Years ago, when I first starting "collecting", I bought stuff I liked and/or loved. It took me about a year to realize that what I had purchased probably had not much re-sale value, since there were no "name brand" makers in my "collection".
Well, I guess I musta had good taste in knives, since I came out whole when selling all that I had acquired, in order to "invest" in a collection of knives. I still have some knives from those days 15 years ago, and what is left, well, THEY are the ones I will NOT recover my money on - only about 10 left, all low value anyway.
My buying criteria changed as I began selling the first of my knives. From simply liking and/or loving a knife, buying now required a three-fold criteria:
1) I must love the knife; (Let's face it, sometimes this #1 controls!!)
2) I must have a pretty good idea that I can sell the knife tomorrow for at least what I paid for it today; and,
3) Beyond #2 above, the knife should have decent investment potential for the years to come, to the extent research can determine it, coupled with my own gut shot.
It takes time to follow the market. The forums, Ebay, online dealers - And they must all be followed regularly. And following the market is the only way I can come up with my own gut shot on the potential a knife has to escalate in value, beyond my own taste.
Now, to answer the questions about selling knives. Because of the criteria I use in buying, I build an investment collection that I love - and I display it, too. When it"s time to sell a piece, I don't ever sell it for less than I paid for it - no need, if I have followed the three criteria above. And generally I profit handsomely, because the knife usually came from someone who either had to sell or someone who dumped a knife to buy another.
Bottom line - I never NEED to sell a knife, so I am in a better position not to lose money. I think everyone has their own reasons why they buy and sell, just like they have their own personalities. And somewhere along the line, we all make contact, have fun in the mix of it all - and share the common trait of absolutely LOVIN' KNIVES!!