Chris,
That's a good choice on the 50mm f1.4. Although I do use some AF gear myself, I love the MF Nikon equipment so it's good to see others getting into it. As far as something like eBay goes, here are some general rules that I follow for myself:
1. Research the lens beforehand if you don't already have experience with it. There may be several versions (F, AI, AIS), so make sure you know which one is best for you and how to tell the difference between them.
2. Get an idea of the new price, provided the lens is still on Nikon's list. B&H Photo and Adorama are two good sources of information.
3. Follow a few auctions, if you can, in order to see how the used prices are going. If you take some time to look around, you may find a bargain and may be able to use "buy it now" to purchase before the bidding starts in earnest.
4. Set an upper price limit and don't go over it. You can usually pick up a nearly new lens well under the new price with a bit of patience. Usually, you will get the lens caps, but also you might find one with the box and a few extras (filter, lens hood, etc.). Occassionally you can get a really good deal on something brand new.
5. Have a standard list of questions for the seller (lens age or serial no., condition of optics, any dust inside, condition of barrel, etc.) and feel free to ask any other questions that become apparent from reading the item description. Check the seller's feedback. Avoid those with excessive negative feedback.
That's about it. If you have no experience with eBay then go through their extensive literature on the auction process. Also, try and learn about how you can pay for your winning bid. There are numerous options to consider. However, after doing your research, you may well find a good price at your local dealer or one of the mail order suppliers. Hope this helps and good luck. Most of all, enjoy the photographic experience!
Greg Lumpkin