Hi Donald,
Impressive collection indeed!
If you need the money, you can indeed find reputable hobby shops that will buy the kits, usually paying about 50% of what you paid for them retail, but $10, $20, $50 here and there can add up so you’ll get some money back and some cash is better than nothing. Some online hobby shops will pay for the shipping, but they have to make a profit too so you can negotiate for cash or more for store credit, but since you want to get out of the hobby, take the cash.
If you aren’t in it for the money, then find your local Veterans Administration Hospital and they will accept plastic kits (no resin) for the Veterans and the troops if you donate free to them. You can get a receipt for your tax purposes (which can be quite a good return from the IRS). The VA Hospital should have even more connections to Veteran charities and museums to donate your built kits to so that they will have a decent home. You can network that way and make sure that your built and unbuilt model collection isn’t hauled off to the dumpster and the landfill. The VA Hospital has vans that might even come to your house to pick up your models. I’d call them and see how you can help the VA Hospital and then from there you can hopefully and probably network with reputable Veteran charities to donate even more (maybe even resin and PE to expert Veteran model builders). I doubt that you can sell your kits to these charities, and do make sure that they are reputable because there are Veteran charities that are total frauds. You can make friends and hang out with these folks because it’s a medical field so these VA medical folks should have an ethical and moral standard to follow (to take care of people).
Auction houses might take your kits and sell them in bulk and pay you a sum, but that’s iffy. You can call around auction houses as these days, some people will sell nearly anything and your kits look like they’re kept on good shape.
The eBay route is slow as most of the kits on eBay are now sold from China. It’s best to hire someone reputable to sell your kits on eBay so you don’t have to manage the auctions and the careful packaging of the models if that’s the way you want to go, but there’s no guarantee that people will buy them and you might not make a lot of money as it takes time, money, effort, and care to ship each and every (built) model to each and every seller.
I would not go with Salvation Army, Toys for Tots (Marines), Goodwill, etc. as they donate to young kids and model kits are for ages 14 and up, and even at age 14, I totally blew chunks as a kid modeler. It wasn’t until I was in my late 20s that I got decent at building model kits.
So there are several options to get the ball rolling, Donald, and just remember, when it’s FREE, there are people who will take it! (I’d go for the tax write-off though…and that’s through the government).