Part of the issue here is that player vendors and player run auctions are part of what makes UO, UO. But if EA could provide a global vendor search engine, similar to the one run by the-extremely-popular-site-that-shall-remain-nameless but covering all of Sosaria and legit, and maybe searchable in game at various locations, that would rock.
One of the biggest benefits is it would place vendors on that remote island in Felucca on almost the same footing as the prime locations in Luna. The problems in getting to these locations would still make the luna vendor houses the prime locations, but at it would ensure that a well stocked vendor, with good prices, in just about any location, would be viable.
edit: As for pets, you should be able to deed them and sell them on vendors. Just have deeded pets count toward stable slots to prevent it from becoming a way to store extra pets. You would also need to have some way of viewing the pet, so I would suggest a right click view option on the context menu which opens up a viewing window (a bit like a tmap) would be ideal.
