It's a simple matter of people low-balling over the internet....something that usually doesn't happen in person.
When I lived in Virginia, I used to do a lot of trading, buying and selling on vaguntrader.com. I'm not ****ting you, I had a guy offer $250 on a gun that was priced properly at $700 and ended up selling for a little less than that after negotiations. It was pretty common for jerkoffs to offer half of your asking price, and out of the 10 guns or so I had on there, I think every single one had an offer about half of my asking price. People would do retarded **** with their trade prices as well.....a couple times I had people actually value their gun in trade as high as a NIB retail gun, and one time $20 higher than storefront prices. Crazy.
A lot of people think their gun should go for retail when sold used, but then they only want to pay half price for other peoples' guns. A lot of people also are complete ******* retards and don't have the slightest clue on how to barter (Hint: emailing "let's cut to the chase, give me your bottom line" isn't the way to do it. I'm a private citizen, not a used car dealership. That line doesn't even work at car dealerships). So, it's just the nature of the beast.
Lay out the conditions of the arrangement in your ad very clearly, and don't argue with people over it. If they're completely off their rocker, don't create a dialogue, create a monologue so they just go away. Something simple like, "No." will usually work. Be very specific and clear in your correspondence as well.
Mags definitely add to the price. Don't put up with retards and low-ballers. Just tell them, "No" and move on.