If they use the assets you created, file a DMCA against them.
Join the hiddendevs discord server and file a scam report against the user (to help others avoid the same fate).
If there is communication about it on Roblox, you can also try reporting them to Roblox.
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I don’t even think buying/selling games is allowed. That’s another rule that he’s broken, and you could report that to Roblox.