When I worked retail, we needed a 30% - 40% margin to cover costs. I'd think a dealer of used games could probably get 10% - 15% of that margin on retail market above FMV, but you'd still take a huge loss. Nothing against retailers, either: no matter the discount, that's a lot of capital to tie up, especially with moving expenses. It's not like a drop shipment from a manufacturer.
You're on the right track, discussing with family. I'd sell them off slowly, word of mouth, to your league friends. Maybe even invite the Capitol Corridor league over for a friendly. Sell local first to people you know (you will also still be able to play them), then here & CL for retail, then consider batching the rest when you do have real deadlines.
I dislike mortgages too, but it is satisfying to pay them down and, as noted above, the rates are way better than a bulk pinball discount.