I post a lot of trade only posts. The reason? Well let me give you an example or two.
Paid $140 for a 30-06 Mauser, bad condition. Honestly, that's what it was worth. But I traded it for a brand new SCCY CPX2 that was least at least $200.
Paid $355 for an absolutely brand new Glock 43 just a month or two after they came out. Traded that for a Glock 26 with a sight and trigger upgrade in almost new condition.
Point being, sometimes I have something someone else really really really really wants. Or they're really really really bored with their current firearm. So I trade, and I get a heck of a deal out of it. If I sold that 43 for cash I'd of got $400 or so. But the 26 I got is valued at around $440 around these parts. The Mauser wouldn't have went for more than $175, but that SCCY later got traded in to another gun that was worth even more than the SCCY was.
Trade only is a way to get ahead, and to limit the amount of cash involved. Plus, if I trade one gun for another, I have a reasonable assurance the other person isn't a prohibited possessor, and if they are, a leg to stand on in court hopefully.