Reputable Apple device reseller since I can't access my purchases.

On a single Apple Account you can have 10 devices associated with that account for purchases and media (no more than 5 computers).

View and remove your devices that are associated with your Apple Account for purchases - Apple Support

As for a reputable reseller and how much money you will get, that is going to take research on your part. Your options vary widely from selling on third party marketplace sites, third party trade in sites, and even the Apple trade-in where you would get a Gift Card for future purchases. There are pros/cons to each approach. I don't think you will find a specific endorsement on a Support Site such as this.

Also note that there is a difference between an Authorized device and an Associated device. Authorized devices are specific to computers with the limit of 5 and iPhone/iPads don't count in the limit of Authorized devices. To remove Authorized computers, you have only 2 options and that is to Remove all devices or remove the Authorization from each computer, not from another computer.

Authorize or deauthorize your computer for Apple Account purchases - Apple Support

If you are still unable to remove a device, you can contact Apple directly, and I have seen posts here where they were able to access your account to remove them.

Contact Apple Support - Apple Support

unxrules wrote:

Thanks for the reply. I read the Apple docs on removing devices. The issue is that I can't remove the grayed out devices, since some of them are dead and I'm not sure where I put the ones that *may* still be bootable. I have MANY devices and I ended up moving older ones to the basement/attic, etc.. A couple weeks ago there were 5 grayed out, then it increased to 7, then 8 without me changing anything.. I can't remove them. Thanks again for the reply.

There are two different things here: those devices that are authorized for media and purchases, and those with Activation lock set.

For the media and purchases, you can perform a global reset once per year: Authorize or deauthorize your computer for Apple Account purchases - Apple Support

For Activation Lock, you can relinquish the ownership lock from iCloud for any computer or device associated, and at any time.

As for consignment or bulk or surplus sales, might want to see how much the gear is eve worth — and whether or not it works — before attempting sale. Prices on most vintage and obsolete gear drop off toward zero pretty quickly, and online sales have substantial risk for the buyer and which pushes the prices yet lower.