There's no "unfortunately" about it. This isn't an auction house. If you are selling, you list the price and you sell it to whoever says "I'll take it." If it's for trade only, you still list what you think the trade value is so other's can look at their trade items for accurate comparisons.
IIRC, you don't "have to" accept a full price cash offer, if you are ONLY wanting to trade - and as Mac said, the price you list is to help others evaluate their trade in comparison - although I would make it clear in the listing that I was just putting the value for that reason and not accepting cash buyers.
This is supposed to keep folks from offering their NIB Jennings/Bryco .25 Auto as a direct swap for your LWRC piston AR, or similarly valued item. Not sure how well it works though, lol, as human nature being what it is, you will always get those who think their stuff is worth more than the highest price ever paid on GB for a similar, while yours is worth about 50% of that value.....