Filets are great, but still may contain bones, so beware. The fish should have a clean look and smell to it, so ask to smell it before you buy it. If the fish has an ammonia smell, refuse the product. The filet should also be smooth, not slimy. With crabmeat, the same procedure applies - if it has an ammonia smell, pass it by.
Scallops are interesting because they smell odd most of the time.
The scallop should be opaque or occasionally golden. Graying is not acceptable. If you're not sure, they're probably fine. They are slimy and they smell. Otherwise, they have little odor and tend to take on the ammonia smell the longer they sit.