How do I price a lot that I want to wholesale?

[Michael Johnson]

I'm new to real estate investing and I have a 7,000 sq ft lot that a motivated seller wants to sell. However, I'm not sure how to go about figuring out what to offer him for the lot. I have looked on Zillow but I'm not sure how accurate that is to use to gauge a pricing offer. Zillow says $18K and the  seller intimated that he wants $18k, so I know that I can't go by Zillow on that. So once I figure out what to offer that he will accept, I'm hoping to assign the contract for at least $10k more to one of my cash buyers. Any advise? This is my first deal.