Ever wonder why office-supply stores will offer a ream of paper or $3 for empty inkjet printer cartridges?

It turns out there is a booming trade in empty cartridges, which are much sought after by the original manufacturers, such as Hewlett-Packard Co. and Lexmark International Inc., and by companies that refill old cartridges for resale.

"There's a world market for empties," says Jim Forrest, an analyst at Lyra Research, which tracks the printing industry. "Prices vary day to day, just like for any other commodity."

Cartridges that fit popular printer models usually sell for about $7, Forrest says.

This cost is a key consideration for companies that refill and resell the ink tanks, such as Cartridge World. Forest says refillers figure on spending $7 for the container and $1 for the ink, which pushes their cost close to $10 after including labor and packaging.

That doesn't leave much margin for profit, considering that refillers try to keep their prices well below the price of the brand-name cartridges, he says.

Even damaged cartridges are worth something.

Ray Kendall, a Cartridge World franchisee in Grapevine, Texas, has a cardboard box in the back of his shop labeled "bad cartridges." Some have fouled print heads, and others are missing the tiny plastic tab found on the underside of ink cartridges. But they don't get thrown out.

"I sell 'em to the remanufacturers for about 20 cents each," Kendall said.