The cavities should be clean with no roughness or dings. The sprue plate should fit flat to the top of the mold, and when you close the mold and hold it up to the light, you shouldn't be able to see any light between the halves. If you do see light, it could be because the halves are warped, or because there is somthing preventing the mold from completely closing. If the later, you might be able to fix it, but unless its a really good price why bother.
