Vincenzo,
I think your Sam's guy is correct. Not sure why your pork ribs were marked "select" unless it was some sort of "branding". I don't have my NMPA guide here at home, but as far as I know, only beef is graded by the USDA on the system you refer to. Another side note is that "choice" is not necessarily the best kind of meat. The highest USDA grade of beef is "prime". I heard they do sell "prime" beef BBQ cuts such as briskets, though I've never seen one, and would not spend that kind of money even if I did see one!
Here is a link to the USDA guide for buying meat:
http://www.ams.usda.gov/howtobuy/meat.htm It explains the grading methods for different types of meat.
As far as the ribs are concerned, look for bright pink or red meat. If it is greyish or greenish, avoid it. If you are allowed to smell it, it should smell like pork blood. If you get a "twang" in the smell, for lack of a better word, avoid it. If the pork is cryopacked, and has a pack date on it, I would avoid anything that was packed over two weeks ago, unless it has been frozen, but freezing is a whole new ballgame.
If you are buying spareribs, look for ribs that are designated 3.5lbs and down. This means that the slabs should weigh 3.5lbs. or less. Simple, huh! Your Sam's guy may or may not know this spec, but 3.5 and downs generally come from younger pigs than than 4.3 and downs for instance. Younger pork usually equals more tender! I am of the opinion that that any cut can be turned into a tasty meal, but the better your information going in, the better your BBQ coming out!