When you enter a compeition to win alcohol online, there's usually a 'tick this box if you're over 18' type button before you can enter the pages. Not very secure but I guess you'd need something similar for proving people are 18 on an alcohol site.
I work for a wine company and one of the ways they do it (based over the phone) is that they don't take Visa Electron or Solo cards as these can be given to under 18's, and also the T&C's witha tick box.
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as above, do not accept solo/electron and force payment via mainstream cc's PLUS a stringent T&C with an acceptance and of course showing 'positive assent' when they check this bix will aid...
maybe more requirements legally but I will step back for the legal eagles now...
You need to speak to your local council. Most who sell alcohol online will already be covered by an offlicense, or sell wholesale, if you are just selling b2c online then i admit, i have no idea, it doesn't fall into any of the licensing categories i'm aware of.
The local council are responsible for all licensing issues, so i'd give them a call. Good luck. What kind of stuff are you looking to sell, specialities?