whole-selling and closings in quick succession - Question

[Ed Tamayo]

Hello, 

I have been investing for a couple of years and run into an opportunity and I have a situation that I suppose whole sellers see everyday:  

I will buy a house from A and I will sale it to C. (So, I will be the B in a A-B, B-C pair of transactions)

I will pay cash to A and, for simplicity, say that C will also pay me cash. ( I don't want to muddy the waters with legal issues around loans)

I want to make these closings on the same day or with very limited time in between. 

The question is: How can i legally enter into  contract with C to sale C the house if I still don't own it until the first transactions closes?

Looks like wholesalers do this all the time so please enlightening me. 

Thanks