If this is fresh, there should be blood and an open wound. Was the pleco bleeding? Is the fin still attached? If the wound happened a few days ago at the LFS
, there would be an open would, but no blood visible. This maybe an old wound if you can see skin developing over the area and you do not see any open sores. (FYI
--The fin you are describing is the right pectoral fin.)
How large is the tank? Common plecos can grow quite large over time and the "sharks" (depending on species) have the potential to be a foot long. Since you are new to the hobby, I suggest reading what you can on caring for your fish. Get some books out of the library and try to identify all your fish, that way you will know how large they can get and proper diet. The internet is helpful, but can also be VERY unhelpful and contradictory.