I agree with keeping an eye out in the classifieds...hopefully on more than one site. Always make sure you get pictures of the scope...looking for obvious damage to the scope tube, turrets and scratched objectives.
Having said that....maybe you could shed a little more light....you say you want to shoot targets at 600 yards and beyond. Are you punching holes in paper or metal targets? Would you possibly using it to hunt varmints and perhaps big game in open country?
Thankfully, these days, we have many good scopes to choose from. A feature I would recommend, especially for starting out, would be to have a zero stop. If you are just starting out and learning to dial in your dope ( predetermined adjustments for a certain range and wind conditions for you particular ammo ) it's really easy to get lost with how many clicks and how to get back. I'd also recommend sticking with a SFP ( second focal plane ) scope. The reticle will stay constant and it will help you concentrate on your shooting mechanics.
You will probably get tons of excellent advice on which brand or model of scope to go with. Ultimately, you will have to decide....the internet is awesome for gathering information...but you can't get behind each scope on the internet. Look at features, reviews of optical and turret performance, warranties and if there has been a consistent issue with the particular scopes you are looking at.
I will throw one scope out for you to consider. That would be the Vortex Viper PST SFP. This would stretch your budget by $50.....but you will have an outstanding warranty and a company that really takes care of their customers. It also has a zero stop feature and exposed turrets, which are very handy for making adjustments. Here's a link to what I was thinking, the moa version:
Vortex - Camera Land NY
Good Luck....you will get a bunch of recommendations....take your time and research the advice you were given. Realize....we all have personal preferences and those preferences may be reflected in our recommendations...it's human nature.
Wayne