I think it would be a good idea to choose how many of an item you want when you are buying something. Some users have a lot of the same item in their shops. It is easier and faster to select the number you want instead of buying each one separately.

I think that feature could be abused when an old item becomes needed for a plot or an achievement because one user could then easily buy all items and sell them for an absurd amout. And people usually don't need a larger amount of one item, so it wouldn't be that useful. So I'm against it.

The first thing I thought of was the sort of abuse mentioned. It's just a really unfair advantage if you get there first.

Maybe adjust trades to take more items instead? Say you still can't have more than 10 unique items, but you can add more of each type. For example, you could make a single trade with 50 steamworks trinkets and 20 lounge trinkets, which would be faster than dealing with 7 separate trades.