I think $300 would be very upper end for a used Legend SL. But, you can check the online E auction place and check the box on the left of the screen that says "completed listings" and see what they sold for last week.
Just FYI, I have two used Hortons and the most expensive one is $329. Both have new scopes and strings on them. Both were checked and completely gone over.
I've shot the SL. It is a nice bow. My only complaint is the safety. I'm not really fond of the way it goes from safe to fire. It's not a positive click. One thing to watch for is the limb pockets. Look closely at them before you buy. If you have any questions give me a call.