Can you get a refund on Roblox items bought by mistake?

Mostly no. Roblox doesn't refund Robux spent on items, game passes or dev products just because it was a mistake. If it was bought without your permission (someone else on your account, a kid on your card) that's different, report it to Roblox support with the date and item name. Worth a ticket either way, the answer is just usually no.