Farming produce is seasonal, so seeds are hard to find after the planting season starts, but easy after harvest, and the produce itself becomes abundant right after the beginning of harvest.
When it comes to the stuff I'd actually want to buy (after the initial purchase of seeds) I'm not sure I've seen any turnover, but I haven't looked closely. It's supposed to be subject to a turn over, so I assume it is.
There is no reason for any kind of quality progression, as UrW is a rather static world: changes were typically over the span of centuries rather than years for people living back then (except for fire burning villages down, war, etc.). Thus, masterworks item availability ought to be random, although it might be weighted towards larger/wealthier villages (and different weights for different cultures, of course).