If the bike is local, contact the owner and ask them to measure the frame size. On bikes of this vintage it is simply the distance between the center of the bottom bracket and the top of the seattube where you insert the seatpost. I'd agree that with the measurements you gave a 54 cm frame would probably do it. Taking a look at the way this bike has the handlebars set, I would think the current owner is too tall for the frame. Turning them upwards would make it easier to grip if you were too tall. Not something I would want to ride that way.
Given the hassle of shipping a bike and not being able to look at it closely I would hesitate to buy a bike from eBay. Where I live, Craigslist seems to be the home of people who list bikes at ridiculous prices but Facebook Marketplace now has a lot more choices and often at more reasonable prices.