"Sitting on the floor, 'Supposedly it runs'" leaves a big wide gap. The brother wanting one and then backing out, would raise a big red flag to me. As it sits, I wouldn't go over $100 on it. If they get it running, depending on a compression test, it could be worth up to $500. (Even if it starts and runs, doesn't guarantee that it has good compression, nor does it prove that the block is not cracked or otherwise damaged.
1950 F5 Stake Truck Flathead 8 "Edith" FOR SALE
1951 F1 5 Star Extra (in progress) "Dollie"
2001 F250 XLT V-10 4X4