Firstly, people sell membership upgrades, not pins. Just use the correctly terminology of 'upgrades' otherwise you won't get very far.
Also people use a variety of secret ways that they will not want to let out.
Some people use their mobile phone.
Others use a CC.
Some might sell Runescape Cards. (Imagine if you worked at a 7/11 and got a 20% discount on them, maybe some people can use that as an 'edge')
There are a lot of ways that people get these upgrades, and I don't think they would want to share how exactly they can get them for cheap, but the 3 ways I've listed are some common ways.