sizing of a Giant Bike

Hello,

I am hoping to go look at a bike I found on Craigslist. I live very far away from any shops that carry cx bikes, so I ned to go this route. While traveling I had found LBS with a Giant bike in size large. I tried it out, and the helpful who manager said it looked big. No stand over at all. He said the ML size would not be much different, and that a Medium should work, but they had none coming. I am 5'10" with a 32 inch inseam. Has anyone dealt with Giant that might know if this sounds close to right?

The used bike I am hoping to look at is a medium.

Also, the bike is a 

2015 Giant TCX Slr 2 with 10 rides on it. Selling for holiday cash. Any input on the bike itself? I'm obviously a newby and don't want to go astray with my money.

Thanks!

Jon