You can’t enable a single gig to only pre-existing buyers.
The ToS expressly state that gigs are user generated content by the seller and there it explains what constitutes UGC.
Also, “Sellers warrant that any content included in their Gigs shall be original work conceived by the Sellers”
A tip isn’t UGC or original work.
Don’t assume someone knows everything because they are a TRS. Plenty of TRS have stolen work, claimed others’ work as their own, had Gigs up that breach ToS and just weren’t caught. It doesn’t by default mean they’re following the rules. It means they got away with it.