I noticed that the Trading Post stacks materials together based on price. So If I want to buy 100 whatever ore, I get to see the total current price something like this:
- 36 whatever ore for 16 copper a piece
- 64 whatever ore for 17 copper a piece
When I hit buy, someone else might have placed an order already so the current situation has changed and the result might look like this:
- bought 10 whatever ore for 16 copper a piece
- and a refund for 26 × 16 copper + 64 × 17 copper
I suggest that the handling of these stackable items get processed as lots like most other MMOs with an auction house do. If I sell 88 whatever ore, it can only be bought as a lot of 88 whatever ore.