The others have you on the right direction on the bed length. Something else that a buyer will often want to know is the swing over the carriage. You measure it the same as measuring the overall swing over the bed, but do it with the carriage run forward and go to the 'legs' of the carriage rather than all the way to the bed ways.
That said, what brand and model lathe have you got? That will tell a potential buyer alot, but other things to include are --flat belt drive -vs- gear head----does it have a thread shasing dial----standard and/or metric tread capabilities, etc, etc. Too, what accessories are included- Ie follower rest, steady rest, etc, etc.
Lastly, where are you located as I am looking to upgrade mine if it's something I might be able to use, the price is right, and the location isn't too far from NC.