While it's obviously not entirely intended, the rules as outlined on the USGS site (I can't find a full "terms and conditions" document anywhere) imply that not only are there no practical obstacles to transferring the pass, but there are no de jure rules against doing such a thing either.
The important rule from the questions seem to be:
Who is admitted with an Annual Pass?
Each Annual Pass admits pass owner/s and passengers in a non-commercial vehicle at per-vehicle fee areas; and pass owner + 3 adults, not to exceed 4 adults, where per-person fees are charged. (Children under 16 are always admitted free).
Note: Photo identification will be requested to verify pass ownership.
and elsewhere "pass owners" are defined as:
Pass Owner: Person or two person(s) whose signatures are on the back of the Pass.
As the pass owner are explicitly described as the persons with signatures on the back of the pass, and there are no rules about the relationship between them, or how the pass is transferred between the owners, it all seems fine.
Images on the internet seem to show the words "Non Transferable" on the back of the pass. Without any more detailed rules however, it is not clear if this simply means that there is no means to get the owners changed, or if it means that the "original" owner cannot relinquish ownership.
Note that there may be specific, unpublished T&Cs, or even federal law prohibiting what you plan. Although if it were against federal law, I would expect ebay to remove such listings.