you are correct that anyone can then waltz in with a visa gift card and use it however the responsibility for that then squarely rests on the person who is either usin the card or those that gave it to them an I am 100% satisfied that I have done everything I can realistically do to prevent little johnny from gettin his hands on a restricted product.
No. A minor is not lawfully responsible for their actions and as such cannot form a bonding contract. What you are doing is providing a way to generate a profit for your pocket and if Dirty Ernie shoots his little sisters Eye out then you have culpability. Because there's money in your pocket. Potential liability Compensation. How to ensure that is a) simply do not sell them online, b) Get prepared for the litigation c) create a physical location that demonstrates due diligence on confirmation of Age of Majority.
Too, when you come to ASC looking for people to pick over your site the replies you should make are, Thank You. Siggy Poo for example but not solely, told you he tried and then said screw it. He paid you an additional courtesy of telling you what the fkn trouble was. Say "Thank You"
Starting this Gig will be frustrating. Don't kill it before it leaves the nest. Asked for Criticism don't bitch when replies aren't all rainbows and kittens.