It's a bit too hit and miss to be anything other than a rough indication of what the seller may be like.
I leave feedback for every transaction, but I have no idea why other people can't be bothered, especially when, as I do, someone has responded to a 'can someone pick this up for me' type thread. I have picked up numerous complete bikes, parts or frames for people and often hand delivered them, only about 60% of the people that I have provided this FOC service to have bothered to leave feedback for me. Not that that will stop me, as I am just a sucker for helping a fellow RB addict out.
I also always leave feedback.
And most people generally feel more comfortable dealing with members with good feedback.
I wouldnt ask a complete stranger to watch my bike while i went into the shop no matter how trustworthy i felt about them.