I have a feeling you can't refuse to accept paypal... though if it is collection only, that may be an exception. Just don't take a paypal payment then let them collect or to organise their own courier; if having it couriered 'you' need to have the proof it was dispatched.
The low bid matter is straightforward - if you have set the start bid at £50 then no bid below £50 will be accepted, any bid above that will be though.