The OP's original question can be viewed in a number of ways.
My take may be somewhat naieve or over complicated.
Any item that is "gor sale" has a number of issues that leadd to a price being attached to it and also that being an indicator to a purchaser of what they would need to pay to buy.
Such is trade as a simple way forward.
So it should be easy to watch commerce tick along.
In the case of here on TP, and in the time I have been a member, there have been the obvious pro/semi pro traders now weeded put. I have bought items but not sold. I don't sell as I have grand-children and nephews and nieces who are also into photography, so I either buy and keep or 'cascade', kit I no.longer need or I buy relatkvely riskfree as presents for 'my kids'. At this time, we are still in possession of everything sourced through TP or I have broken it. (I seem to drop Speedlights on a regular basis).
So the next and more important aspect of the 'trade' is value. In setting a price, the seller seeks a return on the piece to be sold, based on the value they determine, and, as the OP alludes, a price. That price is, in many casess, above what the item is actually worth (as will be seen within a short time when "open to offers" appears against the thread etc.).
So now, in the mist of time comes the invisible but often misused "cost".
This item cost so much..... when price and cost are 2 different things.
Cost is relative and as this thread touched on, buying to sell for the same price is not really zero depreciation but a means to limit cost.
In the bad old days I knew car dealers that sold on cars with known problems, or model changes were due etc. Limiting cost.
One poster here gives away worn kit, scruffy but well serviceable, known limitations etc. If a business then cost is factored into depreciation and 'written down' in value and set against margins and statutory overheads. I used to do the same with IT kit. If a camera was getting close to it's expected shutter actuation limit then it is good business to swap out old for new. As the old cab drivers would say "if the wheels ain't turning, I ain't earning". A failed shutter....
So we have Price, Cost, Value, Depreciation - all different.
Back o the original comment....
The simplest thing is that whatever price is asked for, irrespective of the 'story', if the seller and the buyer reach a deal and all issues are clear (description, price, conditionon arrival etc - no problem.
The real bottom line?
Don't buy what you don't need and if you are happy with the deal then all is good.
What happens when you want to sell.... up to you but don't expect 'the one born everh day' to be on TP......... perhaps?