Who's going to be the first to vibe code an eBay alternative for radio gear?

No need. Hams haven't been using eBay for quite some time. We have other gear-for-sale boards that provide at least some modicum of assurance of who you're dealing with (via callsigns and semi-reliable ways to confirm someone's email). The feeling is, if you get scammed by someone and you have their callsign, you can call them out for it on some of these ham-public forums and put them on blast. Others will be less likely to deal with the ne'er-do-well.

The sites are not at all "cutting edge" or even aesthetically pleasing. Like most of ham radio, it's functional and that's about it. Aesthetics are almost non-existent in the hobby.

eBay serves (served?) a wide, public audience. Hams don't need that wide of an audience, just a targeted audience. I'd venture to guess QRZ.com probably has the most buy/sell traffic, then older, has-been sites like eham.net and swap.qth.com catering to the old timers that don't want to follow the traffic trends that left for other sites.

The thing with ham radio is the population is quite small to begin with. The sheer number of hams is like 1% of any population no matter how you slice it. Country, state, city... and that's just those that are licensed. How many of THOSE are interested in buying gear online or shipping gear when selling online? Not really even the majority of the small community. THEN you further cut that down because hams are a stodgy crowd that resist change. So anything new often has a long run up for adoption. Even longer to become a known entity in the community.

Surely something else could replace QRZ.com eventually, but no time soon that I can imagine.

That's the trivially easy part. The hard part is convincing potential buyers and sellers that most participants are legit and not trying to scam people.

Software is worth nothing anymore in many cases.