can a buyer 'click buy it now' after bidding has started

Not usually, the buy it now payment is taken out by the first bid at the auction start price and once it's gone then it's gone.

The BIN has no gone as you have bids and did not set a reserve,so will be obliged to sell to the highest bidder at auction end.

Cash is safer for collection items,although you can not refuse pay pal if they insist on paying that way

BIN is generally a complete waste of money, likewise with Reserves (Very Expensive, minimum fee £2.50).

If you don't want an item to sell for too little, start at a price you would be happy to receive.