I would suggest going to see it and taking a torch and hammer plus note book and pen along with a camera , do a full inspection of it what's good and what's bad, check its chassis all over plus bulkhead these are the main areas which get rust as the rest is basically aluminium , try find out its history ask the owner questions about it , dies it have a v5 what year is it, why you selling it etc how much dies he want fir it ,
All thus had to be found out before you can even start to consider a price.