Be very careful getting a 50mm prime off ebay there are LOTS of different nikon versions going back years! The latest one Nikkor AF 50mm f/1.8D is very cheap brand new. I found that prices on ebay for this version are actually more expensive than buying it from a proper retailer. I got mine from jessops.co.uk for £76 delievered. (i used a voucher to get 5% off the already cheap price). If you go here here
you can find a review of the lens i have got! Very sharp believe me! If you are interested in the older 50mm primes then go here
Should cover all your potenial purchases. Oh and final thought the latest one as its new from nikon comes with there warrenty (handy).