Where can I sell my Magic: The Gathering cards?
To get the most money for your cards, I recommend that you do the following:


Separate out any valuable rares -- anything worth more than a few dollars -- and sell them individually.
Sell the rest of them in bulk. Generally speaking, commons and uncommons only go for about $.01, unless they're especially valuable in one of the non-Standard formats.


As for where to sell them, you have basically three options. 


Sell them to your local game store. This is probably the most convenient option, but be aware that the local store is likely to offer you less than you could get in other venues, and they're usually flooded with commons and may not agree to buy your old commons and uncommons at all.
Sell them yourself on eBay. This is what I did when I needed to get rid of a few thousand old commons, which I sold in a great big unsorted box. You'll probably get a higher average price per card this way, but you'll have to handle the hassle of shipping yourself. And be aware that big boxes of cards can be heavy, and therefore expensive to ship.
Sell them to an online game store that buys cards. This is probably the least attractive option, since they will usually only offer prices similar to those you get selling to a local store, but you still have to ship them.