buyer tried to scam me

we made a deal that he would go on my roblox and just have it there for trust as i put a pin on my account so he cant trade the items or change the password but he then tries to sell my bubble trouble limited x2 while on my account and fails he then says that "he put it for sale by accident" and i don't get how u can do that 2 times. also we shared screens and when i asked how come i couldn't see ur screen showing u placing my item for sale he said "cause it was lagging idk"