Not sure if that's a problem.
I've asked the following question to a rav before:
Placing an auction on eBay, that ends on shabbos, could be a problem (although its always somewhere NOT shabbos at almost any moment), however a buy it now sale, where people can buy whenever, and you don't care for it to happen on shabbes its ok.
Depending where the machines are placed, it doesn't have to be a d'oraysa that the guy will be oyver by buying.