The only special cover 5e books that became worth reselling, afaik, were Mordenkainen's and Xanathars, the first two. Mord's was actually selling for $500-700 depending on how close to the holidays it was. Xanathars was like, $300 tops. I haven't looked in a year or two though. I have a copy of each that I thought about selling once or twice when I was going through a rough patch, but it wasn't the holidays at the time so prices were a lot lower.A book that will be next to worthless as soon as 5.75 or 6E comes out.
Ive never understood the appeal of collecting special ed versions of RPG books.
The thing about people posting pictures of eBay listings with $400 Buy It Now or Make Offer is that most folks aren't going to pay $400. Just because it's listed doesn't mean it'll sell.
And eBay's fees are ridiculous nowadays... like, 20% of the sale is lost between eBay and PayPal.