person to person sales in WA are for the most part un-regulated (if you know they're not a felon, then go ahead). However, it is common for the person selling to ask the buyer for a concealed pistol license as a safeguard (meaning somebody has already done a background check on them).
Ebay - no guns.
Craigslist - no guns.
Try spokaneguntrader.com (I'm sure there's others)