There are a lot of con artists on ebay which is reflected in the missguided prices you see. There are also a lot of poorly Bay's out there that need a full restoration and the cost outweighs buying one already done properly, so safe to say save up and buy the best one you can afford. You do get what you pay for sometimes but watch out for those nice n shiny buses that are just tarted up to fool you. They are now around 50 years of age so it could be a combination of things.
1. A lot of youngsters today are just not interested in old bay's
2. Lot's of con artists and misdescribed adverts putting people off buying
3. Last few years there has been a massive increase in campers being churned out (especially VW)
4. The knowledge and skill to look after these classics is slowly vanishing
5. People expect mod cons and posh interiors
6. A lot of people can't drive so getting into a bay scare's them off
7. Financially it is now out of most people's reach even if the love is there
8. Possibly the new fuel is having an impact and people are worried about the petrol engine's future
9. Part prices to keep them going is getting silly
10. A lot of people who have nice bay's don't want to sell them
Took me about 10 years of searching and I did fall for a poorly Devon at first which I enjoyed for a little while but the cost to restore it was way more than I could ever afford. I sold it as a running ongoing project, sold my beetle and my golf mk1 gti to afford a nice bus. I only got mine by dropping dead lucky on it and I won't be selling it.