I think we should make a clear distinction between people who flip knives, and people who do not. I see people here giving examples like 'I bought this knife in 2014 for 300$ but will sell in 2018 at fair market value'. This is NOT flipping. Flipping is buying a Shig from JNS, and selling it within a very short period. And I am the first to admit that there will be situations where it is not clear cut like this, but lets take this as an example.
Now for the question of whether to allow this or not. To me, the 'flipping' example is a clear sign of someone acting as a business and/or selling for a profit. In my opinion, this behavior should not be supported by KKF (through BST) as (i) it ruins the opportunity for people to buy a knife at a decent price (I think most of the time this regards knives that are generally available) and (ii) it leads to an unequal treatment of 'normal' vendors on KKF (who do pay a fee for being on here).
Finally, to everyone who is using the 'free market' argument: that may be true for eBay or some other auction site, but I like to think that KKF is a more close knit community where we can help each other out, instead of trying to make money at every opportunity. I know it may be hard to understand for some, but sometimes a little guidance is not too bad for a market.