I've bought and sold a heap over the last few years, both folding and fixed. A few were because I just didn't like the knife itself, but most were due to needing money for bills etc. I always figured that once things settled down I could just replace them. Then on Dec 14, 2011 the import laws into Australia changed and it became illegal to import any knife that could be opened single-handed, which instantly wiped out access to anything with a thumb stud or hole on/in the blade. Suddenly the only knives of that sort that were available were those already in the country and prices started climbing as people began to realise that if you didn't act soon you were going to miss out.
The two knives I suddenly wished I had never sold were my Para2 and the H1 Pacific Salt, both from Spyderco. Both suddenly became almost impossible to find and prices jumped around 50% to boot.
A couple of months back I finally had enough to grab a Para2 that came up, and then last month while visiting my brother we walked into the local hunting shop in the town where he lives and there was a Pacific Salt, so I've been fortunate to be able to replace them. Those two and a couple of others will not be leaving my collection no matter how dire things get.