Help identifying Holley carbs?

I researched completed sales on eBay. The 600 isn't selling at $60 and the 750 averages around $100. The guy says if I sell them he'll give me half the money. maybe I'll offer him $125 for the pair and only pay $62.50. I can keep the 750 as a spare and sell the 600 cheap to someone in need. Sounds like a plan to me.