That would be the right solution. Just a simple season ticket plastic card. Given that Anfield will turnover £80million a year it cant be that hard to update all the turnstiles. Even my local gym has a plastic card pass to let me in through the turnstile at the reception.I guess they automatically enroll on line, I actually looked into trying to sell my Palace seat for the game against Swansea and Chelsea (Anyone fancy it I can look into it further) I can't technically do so though I know some allow paper tickets to be issued the Palace don't.
Maybe it's an idea, I after all would be wary of handing my Season Ticket card over to a random.
It also maintains that family and friend connection if you cant make a game. Youd happily give that card to a family member or friend but you wouldnt give it to some random person from the internet you sold the match to several times a year, having to meet up with the person to give them your precious season card then fetch it afterwards again.