Most of us look for cards to buy on eBay and there are many times where you look at peoples feedback, shipping charges, prices, etc. to make a decision on who to buy your cards from. Sometimes the shipping cost is a little more but the cards are less or vice versa. There is one particular seller that I have purchased cards from a couple of times, and just got my third order from them today.
Bleacher Seats Sports Cards & Collectibles has been a seller with a stellar record of feedback, and consistently reasonable prices for cards and shipping. Shipping charges are a flat rate of $2.15 for unlimited cards, and if that wasn't a good enough deal they put every card in a penny sleeve and toploader. I know to some of us that seems obvious, but most sellers on eBay when you buy a bunch of cards they sandwich them between 2 toploaders or jam them into a superthick toploader.
The turnaround on my last order was 5 days, which I think is good especially considering the care they take in packaging each order.
If you are looking for some singles, definitely try Bleacher Seats Sports Cards & Collectibles before going elsewhere.