When making a shop, there should be an option to put a purchase limit per person. So, for example, if someone wants to give out a free potato to everyone to start a farm, they can mark it so that people can only take one. Then a single person can't take the whole stock!
There are some quick and dirty solutions to this, if you’re trying to give newbies things for free, in this thread: https://skyblock.net/threads/add-a-feature-that-limits-players-from-taking-too-much-free-stuff.143395/#post-1612387 Otherwise, for what purpose do you wish to limit players to a certain number of sales? I love when people wipe my shops out XD