I found the following book very helpful:

http://www.amazon.com/Transformers-I.../dp/0896894452


However this book was printed about a decade ago now so while it was a great help to me then, it may be less so to you now. However it is still a good guide to how rare some figures are and if you were to increase all the prices in it by say 40 to 50% it may still be a useful tool It gives prices of Loose vs MISB as well.

I also still find it a useful tool for knowing what accessories/weapons/parts should come with each toy. There were quite a few G1 toys I bought before getting this book where the seller said it was complete but I found out later was missing many of its accessories.