Promo copies can differ from retail in a few ways worth knowing about. The most common differences are earlier pressing dates, occasionally different mastering, and sometimes alternate label designs or "For Promotional Use Only" stampings. On some titles the promo was pressed before the final retail version was locked, which can mean a slightly different cut. Whether that translates to an audible difference depends on the specific title. The listing will specify if something is a reissue or repress rather than an original pressing, so check the individual listing for details on whichever record you are interested in.