I think there are a lot of good points in this thread, and issues that would need to be addressed with any change in the vendor system. But as a returning player coming back to the game with very little, I have to say that I've run up against the issues the OP mentioned.
I can't buy *anything* from vendors on my shard that a new or returning player needs to get established in the game -- and I don't mean boss mobs, either, or high-end crafting fun. I cannot find everyday resources in quantities I can afford. I can't find mid-level gear. I cannot find seeds to start growing plants, or crafted furniture or the add-ons I need for making my own stuff. I can't find potions to feed my plants. And if I want to make my own potions, I have to spend all night building up NPC vendor stock just to buy the regs to do it. All I can say is, thank goodness for the generosity of strangers -- but after a while I just hate asking for stuff I could buy or make IF there were vendors out there selling the basics.
Pretty much the only things I see on vendors these days is high-end loot, the rare (ie expensive) resources, and collectible rares. But new and returning players aren't interested in those, other than thinking about all the shinies they hope they can afford one day (or deciding maybe they should just buy that 100M gold from one of those websites after all...and as long as they're there, maybe they should pick up some of those resources in bulk -- and throw in a few vet rewards too, why not?!) But who wants to encourage that, when we all know it's a good part of the reason there are issues with the economy in the first place.
Storage on vendors *is* an issue, and gold does have to come out of the economy. But when the only things being sold (or worth stocking) cost 150k and up, it's just propagating the issues. The rich get richer and UO has to find ways to drain them down, and the newbies get bored or frustrated and leave.
Not that I'm leaving, mind you...at least I have the "backup" of a few old rares that I could sell if I really get desperate, which could sustain me for quite a while. But then I'm just folding myself into the same broken system. I'd be much happier working my way up the way I did back in 2002 -- stocking vendors with the basics and selling it to others who need it, at a price that's fair and that we can both afford.