Fun times!
Due to an oddity on value calculations, empty Antimatter cells (Krastorio 2) buy for -1000 credits. Trying to buy one does this:

Error while running event market2::on_gui_click (ID 1)
Item stack count has to be a positive integer.
stack traceback:
market2/control.lua:920: in function 'buy'
market2/guis.lua:98: in function <market2/guis.lua:40>
stack traceback:
[C]: in function 'can_insert'
market2/control.lua:920: in function 'buy'
market2/guis.lua:98: in function <market2/guis.lua:40>

Serves me right, really.