I don't think it's credentials that are in question...it's just that the lack of stamps at a specialists or main dealer count against these cars - it's what the market values for any car, but more so this type of car. Your buyer can meet you and understand that you've looked after it, but to the next owner after him it's just a gap in the history. Having said that, how long have you owned it...18 months? If so, not a particularly long gap anyway - and given its mileage, someone is just going to be paying a low-end price.... and get a lot of car for the money.