I think to sell online you need a bank account, an account with the seller and paperless 'certificates'. You have to register with the seller and give them all sorts of access to your most intimate details. If you have paper certificates you'd have to post them off anyway. You don't want to do all that just by choosing a seller randomly online!
check out your local bank. Often they will have a dealing service. Check out a few and compare costs, walk in - with the certificates, if paper - and do the deal right there. Saying that, it can still take a while for the money to come through (couple of weeks!) but usually you at least know how much is coming.