Take it over to waltherforums.com and put it up for sale. Someone will buy it. But, you'll need to specify if it's a SW99 or P99, AS or QA, what generation and post some good pics. Cash value on them used is about $450 unless there's something special, like night sights or extra mags.
SW99s only came in one generation, where the P99s have had 2 or 3, depending on who you ask. For that gun, honestly, you're probably looking more like $375-400. They're not worth as much as the Walthers. S&W sold them for less, and that carried over to resale value.
Check out www.gunbroker.com and do some searches for 9mm SW99s to get a better idea of what they're going for. I've seen them as low as $300.