1. I would tell them to make a resonable offer.
2. Put a high price and work from there (maybe between $400-$500)
3. If you know what you will let them go for go a little above that. If you would take $300 or so then go a little above that.
I think you really need to ask yourself what they are worth to you. My general rule is something used is worth about 50% of retail (not what they can be bought for new) then deduct a little more for the work that needs to be done. This is the best I can come up with. I'll let you decide which one you will use. I personally think I would go with make a resonable offer and we can go from there approach. Good Luck