I played through a full game with a "fixed seed" for 25 hours resetting shops via time progression at least 12+ times looking for specific items (books and rings) and noticed the following:
- Different shops seem to carry the same stuff at the same time. Armor shops that can carry tungsten shields will all be carrying tungsten shields. Inns that sell Sword books will all be selling 2 or 3 sword books. It's like collusion
- Shops will never seem to sell any other items but variations of a core few. I never saw a cartography, or heavy armor book for sale along with rings that give a bonus to wisdom.
- Have each shop have its own, different, fixed random seed. That way shops will all carry different stuff at different times,.
- Every week when you generate new inventory for a shop, create a new random seed from the last random seed (will produce consistent results). This will ensure that the shop has a chance of carrying different kinds of items than it did the prior week.