I always use gunbroker...to give me an idea of what things are worth....if a member(thats free) you can go to the advance search and see what things sold for to give a better idea what people will pay..... instead of what some are wanting to sell for.
I agree with Ironman. It's more difficult with something that's rare, because things are only worth what someones willing to pay for it.If you go to the advanced search portion of Gunbroker you can see what people are currently paying for items.
You can also go to budsgunshop.com (they tend to have some of the better online prices) for currently manufactured items. Compare that to what you see on gunbroker and kind of average it out. If your selling or buying locally though you don't have to worry about shipping and FFL fees, so you can take that into consideration when looking at a price.