$900 sounds good. The same MacBook on the Apple Refurbished store is priced at $999, so I doubt that you would get offers for anything higher than $950.
Are there any cosmetic defects, such as dents (on the bottom cover), scratches (especially on the edges of the palm rest), etc? If not, then you can format the HDD, clean it all up (rubbing alcohol or something should do the trick) and consider a slightly higher price.
Regards
Raptor