Nice one Willy by getting on the BSES exped. It is sure to be a great trip.
Just ignore the misers that have no imagination. Yes you could sell most of your possessions, but what would that leave you when you get back? Plus I assume part of the challenge is raising the cash – just selling stuff defeats the object in my opinion.
As ‘ourmaninthenorth’ said, work out what proportion of the money is paying for the trip, and what proportion you have raise for charity (i assume this is set by BSES). Try and pay for the trip yourself, but use fund-raising methods to gather money for the charities. It will help if you make it clear that the money people donate will be going to these charities, and is not just going to help you have a good time.