Second hand Canon EF 24-70mm L F2.8 buying advice?

Hi,

I have a Canon 5DMK3 with a 50mm 1.8.

I now want to get a 24-70mm to be more veristile and be able to start shooting events.

What do I need to look out for buying a second hand lense? Any top tips, I've seen a few on FB market place

Thanks