Buying a boat is similar to buying a car - buy from a dealer and you often have better post sale rights and support, but sometimes at a premium. Buy privately and you can more than likely save £ on purchase price, but potentially be on your own after you've bought it.
The key difference for me is that with boats, the issues of VAT and warranty validity are greater, particularly when buying ribs (more so with obviously commercially specc'd ribs like a 7.8 pax). Establishing provenance / VAT status, together with the more robust consumer protection buying from a dealer seems to offer (plus, in JBT's case, goodwill) all add up to a dealer purchase being prudent in my opinion.
It's also worth bearing in mind that JBT tend to buy incredibly well and therefore the difference between dealer ticket price and private price may not be too great. For example, a newer version of the rib Neil has bought, with the same spec, with less engine hours and a brand new trailer (my old rib) was sold by JBT to the new owner for considerably less than the asking price of Neil's here....
ballistic rib 7.8m - 250 etec - £33,000 plus vat
In this case, had you offered to buy my rib through JBT, if you highlighted the engine/seat staining issues with the dealer prior to purchase, you would have found them more inclined to do a deal and rectify them, either because they would have beaten me up on price, taken a bit of a hit on their commission or both - a private seller may not have that flexibility. Each sale is unique, but for me, dealer purchase just makes more sense.