It takes the seller out of the loop Barry, the MD person does the showing round, but TBH the ones I've had to deal with when buying or selling know very little they are salesmen from what I've gathered nothing to do with the MoHo industry at all and therefore don't have a clue what is what or how they work, I've bought one and they did do some warranty work but it was terrible and I got shut fast just scraping out without losing as I battered the price down, when I sold mine they asked what I wanted for it then they add £3k to cover their expenses and fee so immediately you are £3k more expensive so not an easy sell if other similar vans are out there, and the descriptions are largely what you tell them, so as you say what is the point, I can write a far better one that them any day of the week and sell myself, the only reason for selling via an agent/auction is fear of the process (which I enjoy) it is marginally stressful but no biggy if you're just honest and respond to questions, but make sure you have covered all aspects as you might only get one chance to show them.
I found when couples came, one would be enthusiastic and the other a bit frosty, good cop, bad cop so I focussed on the woman regardless as usually the bloke might do the driving and anything physical but the woman would usually actually have to use the habitation stuff and they like it when you address them not the bloke.
I still have the description for my self-build if you like a read