Yes. It is illegal.
To reuse stamps.
The question becomes: have these stamps went through the system? It's only illegal to knowingly reuse them. For all you know someone was going to mail out a lot of things, say Christmas Cards, say Wedding Invitations, etc., and due to wrong addresses or a failed engagement, decided to cut them off and try to sell them.
I mean, we can be probably very sure they've been through the system...
It happens. If your Post Office has any sort of "keep local mail local" instead of sending it to the sorting facility, if it's non-machinable, if it's a parcel, if the stamp is affixed but isn't in the right location on the envelope.
https://www.law.cornell.edu/cfr/text/27/479.163According to that, you can't even remove your own stamp if you massively screw up the envelope before depositing it into the mail.
Will anyone actually care if you remove your own stamp if you haven't deposited it into the mail system? Probably not. Will anyone care if you reuse uncancelled stamps that have went through the system? Yes. But it depends...there's always the few people that attempt to reuse Presorted Standard stamps and they just get it returned to them as Postage Due.
Postal Inspectors don't have the resources to prosecute everything.
But, as to the legality of reselling used uncancelled stamps? My reading of the linked law speaks of attempting to remove the stamp. The
ebay sellers aren't attempting to remove the stamps from the paper, are they?