Seriously speaking, I see no reason to forbid links to any buy-it-now item with multiple products available, perhaps with some sort of limitation either by length of Ebay membership and/or number of sales, along with a reasonable minimum satisfaction rating (which, with Ebay being what it is, should be pretty high afaic.)
[ETA: Indeed, on further thought, I don't think there's even a good reason to impose any specific requirements respecting sellers' history or ratings, except maybe (though not necessarily) to exclude "brand new" sellers with literally no feedback or sales history on Ebay at all. After all, we're all adults (more or less) who can read, and more to the point, there are no similar requirements for non-Ebay retailers - either those with free-standing websites or third-party sellers on sites like Amazon, Newegg or Best Buy - and there's no rational reason to treat Ebay sellers any differently. Judging from experience, it's not like any of the former retailers do much if anything in the way of "policing" their independent sellers or their listings... Indeed, the way things seem to be working these days, buyers probably have more reliable recourse against "bad" sellers on Ebay than on those other sites, let alone independent ones.
Having various requirements for sellers and/or buyers on AT's own FS&T subforum is one thing, since those transactions are being facilitated by this site, but I guess I just don't see any need - or frankly even a rational basis - for having "nanny-forum" type rules concerning one specific, and only that one, unaffiliated retailing website...]
Sure, there might be the occasional attempt to self-advertise, but given the rather, um, fierce? nature of many AT forumites, I suspect there would be more finger-pointing and ostracizing of mere potential violators than there would be actual violations. And it's not like there's any massive downside to even the occasional violation, which would almost inevitably end up coming to light sooner rather than later, bringing down the aforementioned unofficial censure as well as whatever official censure would be considered appropriate (etc, etc, etc...)