At least, the greater ones can't be bought for gold...
I think this can work okay as a
general rule, but it really only works for one of two reasons:
1) Cuz the DM says so (if he controls every NPC in the game, he can just say no one will sell/buy the item just cuz).
2) Greater items are useful to
everyone.That last one is important, because if the fighter doesn't have use for an inferno wand of infernoness and the wizard doesn't have use for the ice sword of frostiness, then gold
totally is more useful to them. Really, all they have at that point is a priceless back scratcher. Now, in a perfect world, these two guys know each other (or are in the same party), so they can just trade their priceless swag and get something cool. If not, they have to hold onto their back scratchers until they
find someone to trade with.
If they're having too much trouble trading, they might
really want to trade it for some mundane gold. I mean, if you have this priceless back scratcher, wouldn't you rather trade it for a bunch of gold, and then spend that gold on a mundane back scratcher, some beer and a few hand jobs?
TL;DR: The DM can make edicts like "no one buys/sells greater magic items", but I think that's hard to enforce unless it's not too hard to trade.