I don't buy used scopes that I cannot evaluate personally. However, SWFA does have a section of their website where they sell open boxes and scopes with minor blemishes. They will accurately describe what they offer and prices are good.
For about $500 you can get a new Leupold Mark AR 6-18x40. It is "old school" in that it has only a 1" main tube, no illumination, an adjustable objective for parallax rather than side focus and a now less fashionable but quite usable mil dot reticle to go with its mrad turrets. The glass is good, the turrets repeatable, It is a good basic scope for long range shooting on a budget in addition to shooting groups at closer distances and load development.
Here is a good review. Although this is a couple years old, you can get a decent evaluation from an objective source.
http://www.snipercentral.com/leupold-mk-ar-6-18x40mm-full-review/
Another good single purpose choice would be the fixed power Weaver target scopes, like the Weaver T-24. I think they are priced at about $425 or so right now with most vendors. A lot of entry level or mid level target shooters on a tight budget use the T-24 or T-36. I'd not go above 24x as higher magnification makes eye positioning very rigid and unforgiving when looking through the scope. Even the slightest movement to the side and you are lost. 24x is as high as I can use and stay behind the scope very long without fatigue.
If you want to test the absolute maximum accuracy of a rifle at 100 yards, you need more than 10x. 100 yard benchrest rifles are shooting with 36x and 45x scopes. Such scopes are useless for any other purpose, but even at 100 yards, the top competitors are using magnification that high. Just don't cheap out on the glass, because as magnification goes up, especially above 12x or so, the requirements of the glass quality get higher to maintain a usable image. What is as important as magnification is resolution: the ability to resolve, or see fine detail, like the need to see and aim at the same location inside 1/4 MOA boxes on the target.