Y'sure about that? What if the person is an idiot and buying it for Pinball Wizard (before deciding if they want to buy Tommy) and the Seeker (because they don't know where else to get that song. Ken (Evil Empire Records) said:
Its a fine line between suggesting and preaching. Nobody likes to be preached to unless they are already in the fold to some degree.
Back when I owned an actual brick and mortar type record store I did more than my fair share spreading the word. It was the right venue for it... Its not hard to get a guy buying a copy of Meaty Beaty Big and Bouncy to go for a Fleshtones or Chesterfield Kings record. Pretty much that guidance is what they're there for.
Opportunities like that are fewer and farther between nowadays. I mostly wait until someone asks my opinion.

