I have been lucky and had no shocks when buying from photos HOWEVER apart from asking for lots of photos of all areas you must ask every question that you can think of and dont assume anything. For example I asked if any thing was missing that I should expect to be with the van and found out that one van was missing all the wiring loom!! This info was not forthcomming and it would have been down to me if it arrived without it!!
I think the biggest problem is vague descriptions and deciphering what certain phrases really mean (example.... "signs of useage", "normal wear and tear for a 40 year old vehicle", " motor and running gear will just needs a wake up/freshen up" etc etc)
I think its also worth noting that importers or USA sellers are, in the main, scraping the bottom of the barrel for vans now, especially the lower cost vans IMO