Just some thoughts.
Well first off, anyone that has $10,000 laying around to spend on AR-15's knows that is not a good investment. They didn't get rich by buying Rifles for $1000.00 and selling them for a $500.00 profit a year down the road. Its not worth the time and effort. Yeah you could make some money, but after shipping and handling there goes a big part of profit. All of shipping and handling includes gas, paying FFL fees, shipping fees, advertising, time lost driving to Post Office or UPS then waiting in line to ship it. Time is money people anyone that has tried to run a business knows this. Trying to make money at this is just a poor decision. Guys that have that kind of money typically don't know how to build an AR-15 out of random parts. If they do they are a gun dealer and can buy them cheap enough that their time is not worth building one for sale.
People who think zombies and the apocalypse are real, take off your foil hat already.
For those who are trying to make out like no one is selling there " semi auto military rifles" ( is that better?) look at the rifles for sale section here at THR.
Bottom line (my theory) people are selling them because they have no use for them, they have already impressed their friends with it and now its not that cool, they found out how expensive ammo is, they realized there won't be an ASB for another five years and should sell them while they think they can make a profit.
I am sure some people have sold their personal rifle because they seen they could make some money, but they won't be buying another one, till they get cheap again. Also what drove them to sell it in the first place, that is if they can't get another one at a cheap price.
What gun monkey has the money to buy two $1000.00 rifles and sit on it for a year for a $500.00 dollar profit? I can think of alot better things to invest that money in.