Great, great discussion. As it were I've already posted a pretty comprehensive reasonable post on knifeswap regarding this very issue. In short I went with Leaving the prices up. Mostly and only for that forum/exchange though and it is entirely based on their rules, regs, and whole modus operandi if you will. Oddly enough I usually don't leave the prices up unless of course I forget or get lazy and forgot to mark it sold. That said I've only had a couple times where someone asked me what the knife sold for and I had no problem disclosing that amount. So for that community and situation I voted yes.
Let me say this whole discussion has got me thinking I may change my practice to leaving the prices up. Why I normally delete them (when remember to) is mostly it finalizes the deal and lets others know the knife is SOLD just as an $SPF would be. I also agree with the belief others stated the price sold really is only between the buyer and seller and often that price might be negotiated lower and the price left up doesn't reflect the price the knife sold for. In this world of rules and regs gotta admit it is nice to be so flexible and leave that up to the individual seller whether to delete or not. Too bad our government didn't have more of that right?
Removing the selling prices really helps protect the market if a knife was "drunkenly listed" or let's say emergency sold (for example a BountyHunter Ultratech for $220). Let's say this price of $220 was left up and a noobie comes along and thinks shucks I paid $325+ new but 3 weeks ago one sold for that so I'll list at $230. Now he (or she) leaves the sale price up and now you have 2 listed recently at this price. Another coms along and 1 more and repeats the process this time a bit lower that $220 because his cat is sick and needs the money or a vet bill (

) and now you have a brief market shift. Guess what happens when seller #5 or 6 comes along and wants to get full price plus shipping costs for his brand new Bounty Hunter? He might get lucky but he also might get severely haggled and demonized because .. "hey this knife sold for such and such here and there etc. and you are scalping the market."
What actually really bugs me though is when someone removes the whole listing once a knife sold. Anyone else like to browse the Exchange just to see what different knives people are into and what kind of price range they are in for fun? I've been turned on to some pretty sweet brands and makers I never heard of just by this alone as the general non-exchange forum area is so large anyway. I enjoy seeing what people are into and consequently what they aren't. Forgot just the pricing, removing whole listings suck as then the buyer has no recourse about a knife should the transaction turn sour and also we as spectators don't get to see what kind of cool knives people are into (or not). Want to talk about having a reference for prices and "information", how's removing a whole listing for you? Curious how the moderators feel about this.
Back to just the price though, as much as removing the price has been smooth sailing for me, gotta admit it is quite nice to see what other knives have sold for when I am considering listing one of my own. Thus based on this alone how it has helped me, I'm much more included to leave my prices up in the future.
Lastly, another argument for removing the prices is the pure fun and spontaneity of it all. If everyone removed prices then possibly the prices would fluctuate all over the place making for an interesting exchange. Going back to the Bounty Hunter (limited quantities available) example, with no immediate reference for past pricing you'd have sellers doing more guessing which may look like this.....Seller #1 $315 , Seller #2 $220, Seller #4 $345, Seller #5 $279. Whether a particular Bounty Hunter sells will be SOLELY determined on A) what the seller wants for it or believes it is worth (to him or a potential buyer) and B) what a buyer is willing to pay for the knife (what he or she believes it is
worth to him or her). Thus it really magnifies and simplifies the whole process. After all isn't that what the exchange is all about? Talk about liberty, it really is because what one knife in said condition is worth to one person may or may not be worth the same to another. There's all sorts of reasons and intentions why people sell knives for the prices they do and why people believe knives are worth it to them. When those 2 things line up between buyer and seller that's where the magic happens. Thus not only does this simplify things but it puts an end to (what often happens over on Reddit) demonizing or chastising people for listing a knife at a certain price because they may or may not agree with it. Again, deleting the prices simplifies and returns the exchange to the magic of a sale being between the buyer and seller.