Click the buy now button on any item, then click cancel. Now, type something into the search bar up above and press enter. Then, it quickly shows the confirmation box that you’ve bought the item, and goes to the next page.
(edit) apparently, it costed this guy 11,000 robux. That really sucks. I hope he gets it back.
Thanks for the report. Looks like this happens anytime the Enter key is pressed after the modal is closed. I will fix this first thing tomorrow morning.