I just bought a used Impact. so far, results excellent. Purchased from AoA, They had used Streamlines a few days ago. try their used page.
It's been stated that a regulator is required for accurate shooting. Not always the case. My Marauder can shoot quarter sized groups at 75 yards if I do my part. You just have to spend time with the gun and get to know it. Regulated guns are easier to shoot accurately, but it's not always required.
Used Wildcat's with standard Smooth Twist barrel may begin to show on the used market as well, since the MKII came out. both are excellent guns. If the dealer is reputable, I wouldn't hesitate to buy used. If the dealer is sketchy, you might want to thing twice.