My best friend said he'd sell me his iPhone 4s 32gb that he got last year for christmas, and I know that its in good condition. What would a fair deal be on it?
Check Apple's website. They still sell the 4s and you can get an idea what they go for new. Not sure if 32 GB are available, but add roughly another $100 to the 16 GB phone.
Also a good place to get an idea of prices is on eBay. You may see some extremes of prices at both ends (really cheap or really expensive), but you can generally get a good idea of what they sell for.
What is your friend asking? Better to post that here and see if folks find it reasonable or not.