Worth repairing one that does not turn on at all?

"This sounds like a rabbit hole. Let's assume both are water damaged and no parts on either are any good--that's pretty typical for eBay. So you'll spend time swapping parts back and forth and you'll have no idea which parts are good and which are bad.

You need a known good device in order to make progress here. Get an iCloud locked one and take it apart and use it to troubleshoot the parts. Then build a new one based on results.

Most people try to fix devices before listing them on eBay, certainly the one that has no lcd. If the seller has sold other mobile devices then you can betcha that these have no working parts. If the seller looks like a regular person, then maybe some parts on these will work."