My 2 cents even though this looks like its been covered pretty well already:
Telling the price someone paid is subjective and will only cause tons of market confusion for the reasons below:
Example 1:
Most guys put something up for 50-100 or more than what they're actually willing to take for a bike/part they wanna sell because as we all know "you can always go down but you cant go up" in negotiations. Well, say you have a guy who has no shortage of "folding money" and see's your slightly inflated price and says "I want that, I will pay that because I need it". Now, the next guy who owns the same thing will think what yours sold at is rock bottom market pricing so, (just like you did) he will post it for 50-100 more than what he is willing to take (your original inflated sold pricing). This goes on for long enough and it works its way into a super inflated price and people start to think that this common item is priceless. The guy who originally bought the inflated price item wasnt that search-savy and had no idea the item he paid top top dollar for was a common item and if he would have waited longer he could have gotten it cheaper.
Example 2:
The bikes I buy I prefer to be rusty and crusty and I pay up for them to make sure i can save em the way they are. Well other guys see my bikes and think "ugh thats the worst version of that bike and I wouldnt pay much for it at all because it needs so much work. To me, it only needs grease and a butt in the seat so what I paid, is subjective to my personal taste. I have paid 600 for bikes other guys wouldnt pay 200 for because I prefer them the exact way the other guy would hate to have it.