Check all the normal stuff. Is the tube and the rocker box beat up? If so that tells you how the scope has been treated.
How does the coating on the mirror look?? Ask to take the scope out and star test the mirrors. Remember Coulter makes very poor mirror cells and there mirrors perform much better in a real 18 point cell.
Conveting it to a truss tube will save you at least 75 lbs and allow you to store the scope in a smaller space. GOOD IDEA!!
As for price??? I think $800 is a bit high when you can buy a new Meade 16'' dob for 900. I'd use that as a bargining chip and try to lower the price. $700 is ok $650 is better.
good luck
darren