Prints from scanned & edited photos

I'm new here, but I have a question...I'm thinking of selling some of my
photos on Ebay, and instead of getting the photos made directly from the
negatives, I was going to get prints made from scans of my original
prints that have been edited in PhotoShop to lighten them up some. I
scanned the photos into the computer at 400dpi as tif files, then edited
the color levels in PhotoShop. They looked grainy onscreen at 400dpi,
so I printed some test photos out on my printer (I have an Epson Stylus
Photo 825), and even on the best photo setting, they came out looking
grainy. If I upload them to a site like Yahoo and order prints, or take
them to a place like Eckerd/Walgreens, will they still have the
graininess when they're printed out? I don't have a professional photo
lab anywhere near me, so a drugstore, Walmart or Kmart are the best I
can do for the prints.

Valerie