I like this forum. Bought and sold a good bit here.
Few things about cheepskates:
1) An offer is an offer. Decline or ignore it if it is too low. Always though, be polite, even if irritated. I always ask for offers, but say I may politely decline and have done so many, many times.
2) There is a market dictated by not what one pays for something, but rather by what someone is willing to pay. Used plastic guns are often listed here for greater than retail price on a new one. Ain't happening. I can buy a Gen3 G19 at Aim Surplus for under $300. Why would I pay $500 on this forum even if the poster paid $599 at his LGS?
3) If your gun isn't an investment piece, you'll take a loss most of the time. Think about that when you buy it. I have a safe full of AR's, many bought/built when Barry the Coke-head was President. Worth twice or more then than now. $2000 AR now worth $500 - just the way the market is. Sucks, of course, but I can't get mad when someone won't pay me what it cost to build when everything was inflated. Sell or keep, but don't get mad. A Smith with a lock will always depreciate; a P&R Smith without a lock will appreciate. Know your guns.
4) Low-ballers are part and parcel of sales. Guns, cars, building materials, you name it. My best guess is that the OP would gladly buy a gun for 1/2 of its worth....wouldn't feel guilty, just feel happy he got a good deal. Everyone is human in that regard.
Lastly, don't buy on impulse. Think carefully about how much you like the weapon, how long you might keep it, and what it might sell for when you get tired of it.
I always remember something my father taught us growing up: Always be decent until someone gives you a good reason not to be, and then you should probably be decent anyway.