[MegDreamsofBooks]

With due diligence, Ebay can definitely be a good source for older Kindles. I purchased 2 Kindle 4 devices a few months ago, they came from one of the Goodwill sellers with a solid return policy and I had no trouble updating then registering them. They work very nicely to transfer my ebooks into Calibre, and are old enough I still get color covers (and low risk of a firmware update that blocks the transfer.)

Another good source is Woot, they frequently do deals with piles of refurbished Kindles from recent generations. These are usually still new enough for firmware updates, but also much safer and easier to purchase and register than something from Ebay.

That said, if you continue to buy ebooks from Amazon, you always risk being locked out of transferring them eventually. So while I do have the older Kindles for the few books I can only get from Amazon, my preference these days is to buy elsewhere, and only use Amazon for Kindle Unlimited books which I don't download because they are loans.