i work in a sports collectibles shop and i'll tell you right now, not many shops are buying cards anymore since the emergence of Craigslist and Ebay. those are your 2 best options. you can always go to card expos (if they have them in your area) as well, but online is your best bet. it gets them in front of a wider audience.
also, unless they're really old cards (I.E. the '40s, '50s and some '60s) don't expect to get too much for them. card collecting and production reached it's zenith in the late '80s and early '90s and as a result millions of cards were produced making them virtually worthless. big name rookies, jersey cards and autographed cards are your best bet for a few extra $$$, but even then it's not what it used to be.
there is a magazine called Beckett that is a price guide for cards that can give you an up to date value of what they're worth. you can find the magazines at book stores and sports/card shops. beckett.com might also be able to help you find out how much you should be asking for them. hope this helps.