suggestion: make default shop item count 1 instead of 0

[Former user]

currently buying 0 items says it's "successful" and costs 0 gold, which looks like a bug.

nobody ever wants to trade less than 1 item.

[Houdini111]

Makes sense. I had it default to zero to prevent accidental purchases, but I guess that that's a little too cautious. I'll also make an error for if you try to buy or sell 0 items (both in quantity and in number selected)

[Houdini111]

Implemented in commit 1a95518