And the reverse applies - why should a buyer trust someone they don't know with several hundred quid?
That may well be the way that it's usually done - but it's also the way that people usually get ripped off.
I think that it's time that we did question this method of doing business: We're not talking a straightforward sale situation here, we're effectively talking about an (albeit short term) credit arrangement.
You wouldn't walk into a bank and expect them to advance you money without an established credit record. So why expect a complete stranger to do the same?
It's all about balance of probability: While anyone could default an established member with a proven record is a lot less likely to do so than a newbie.
It ain't rocket science.