You just never know what you are buying or selling unless you can fire it up and see for yourself and even then it might end up having a bad cracked block. I buy them for $25-$100 usually without anything but a visual once over. Sometimes you win but mostly you lose. My main goal is to salvage what I can. I sell short blocks that have been tanked and fluxed for $450-$750 depending on how they mike out. A good solid runner that don't smoke or overheat is worth about $1000-$1500. If it is rebuilt and you have the build sheet on it it is worth $2500 and up depending on specs. These are just general parameters that I use and I have been buying and selling these motors for a while. It is mostly a hobby that mostly pays for itself but every now and then I take a hit and every now and then I make a score. I don't do it for the money but because it is better than golf, bingo, or drinking and chasing women, IMHO. Good luck and in the world of flatheads common sense is going to be your best tool.