How to prevent ppl from buying...

Wouldn't that be the opposite? Your scenario is having max mesos already with an item at 1 meso, while the question seems to be about how much mesos you need to have in order for a max meso item not to sell/in order to sell an item for max mesos.

Yes, what I said was the oposite, but the OP's question was already answered. I just figured I'd state that there are other ways to keep an item from selling. I suppose I could have also mentioned that as long as the price of the item being sold, added together with the merchant's current amount of mesos, is greater than the maximum number of mesos, the merchant can have the item in his or her store without fear of it being sold to anyone.