To bring vendors to the attention of the admins on the market site buyers can click on the 'report vendor' option and I would encourage everyone to leave fair and accurate feedback for the transactions that they conduct. The admins can then conduct an investigation and take the appropriate action where necessary and if everyone uses the feedback system correctly any issues with a vendors product should become apparent pretty quickly. To answer your other question yes it is sometimes possible for the admins to link multiple vendor accounts including previous banned accounts.