If you go to Smithy to buy slime coins, you can only get 10 at once, no more. Which is really strange.
If you close the window and open it again, you can buy 10 more, and so on to infinity, so it's not a problem of limited stock.
Then why can't you buy more than 10 at once?
I would really like to know.
First off its Smitty.
Second, a lot of the merchants will prevent you from going over 10 on any item you want to buy, and if you want more you can always talk to him again right after your purchase, so this is not just on slime coins. This is probably to prevent players from going over than what they originally plan to buy since you can go overboard and realize you spent too much.
EDIT: Stupid autocorrect
IT'S SMITTY!