Since Macs have traditionally (though not recently) only had one mouse button, control-clicking things on a Mac has the same effect as right clicking. This is problematic in Idle Game Maker (or at least in Bunny Clicker), since you need to control-click to sell.
However, due to another bug, there is a workaround. Control clicking on the item will bring up the right click menu, but upon dismissing the menu it's still "stuck" in sell mode until you hit control again. So you can control click, click off the menu, and then start regular clicking to sell.
An explicit "sell" button ala Cookie Clicker would solve this issue. Alternatively, using a different modifier key on Mac (a good candidate would be the "alt/option key" to sell would also fix this issue.