Even if it doesn't look right morally speaking (they're just making money off something they probably didn't work on (but who knows?)), isn't it actually allowed? The seller doesn't state anywhere that he sells the levels, and OpenTomb is an open source project
following the GNU GPL v3.0 license which allows for re-distribution and commercial use.
The only problem I see is that the license the seller uses is just "Standard", while it should be GNU GPL v3.0.