Add velocity controls for the IP address and you'll get a thousand sign ups from a thousand IPs. Limit it by device and you'll see a thousand devices. Flag the images using a perceptual hash and the images will come back superimposed on random pictures of food.
At some point you find that your accounts are now traded as a commodity on underground markets at various prices by teams that specialize in creating accounts, aging them and selling them for x cents. Add enough restrictions to new accounts and you'll find existing shops being taken over and flooded with listings.
And do all of this while still having an "effortless" signup experience for new legitimate vendors.