Financing a buy before closing a sell

[Tom Seeley]

My sister is selling her home in one state and moving to another. She has a contract to sell and closing is May 29. Let's assume it will close without pblms. 

She wants to try to find a new home and close on it ahead of moving out on May 29. Let's assume that's not only feasible but also smart! (Yeah, right...her bro says...)

Let's assume she will buy for an amount she can cover completely with the proceeds of the sell.

What I'm trying to help her understand is: how can she finance the buy before she cashes out of the sell?

All I can think of is for her to arrange a conventional mtge on the buy and then almost immediately pay it off completely with the proceeds she nets from the sell, even tho that'd mean she'd only hold the "buy" mtge for a few days or weeks.

Do I understand the situation correctly? Are there other options?