Sellers not shipping

Level 3
I have had it with the ship to me sick of sellers being able to cancel items after I pay,with no repercussions!OfferUp is charging about 10 percent seller fees but is seriously lacking.i bought a Wii u from a seller to have shipped to me.5 days later seller makes up bogus excuse to cancel.then turn around and sold it to someone else for $40 more...
That is not acceptable. There should be a allotted amount of time that the seller needs to ship the order. There should be a invalid reason for canceling.