From experience with MoD auctions, I can also see a basis of truth in the story. Things do get lotted up incorrectly or misdescribed at sales, including at the MoD disposal sales, but in my experience a buyer getting more, less or something different than they thought is more often than not the buyer's fault, not the auctioneers'. A bike in the back of a truck is likely to have been offered as such by the auctioneer and missed by the buyer rather than anything else.
I've seen a lot of spares come out in the back of vehicles, and I've got a Rubery Owen office trailer that came with new lighting, map tables etc. in the back, but I'm pretty sure the disposals people knew they were in there.
As to the stories of new jeeps, why not? Perhaps they won't have been CKD in crates, but I've seen plenty of brand new and delivery mileage only vehicles go through MoD sales, so why would that not happen in the '40s and '50s? The key to the story, though, is usually the price, forgetting the effect of subsequent inflation.
About the time I was going to Aston Down sales, I picked up a catalogue from the late '40s with prices in. On the face of it they looked cheap, with Bedford 3 tonners making around £25, but work it out in proportion to the weekly wage of the time and it was almost exactly the same as the current going rate for Bedford TKs at Aston Down.
I suspect Vass may have been given the option on those Bedfords; something auctioneers (used to) do to get through a large sale quicker. Instead of putting each up individually, the auctioneer would offer the first lot with an option on a number of subsequent identical lots. The winning bidder for the first lot could then take the rest at the bid price.