I think nowadays because of the membership card everyone seems to buy these. These are obviously stocked, so if the retailer store is out of the stock, so will the trusted members. From what I can see, SMS and paying by phone ect. seem to be more of a hassle rather than giving a membership card.
Just keep checking the upgrade section often. Most newcomers are just looking to scam, so I would look at buying from trusted members like inpherus, DangerousConformist, etc.
Is this directed from your IP? Because I thought you could sell as much as you want if it was using Runescape Membership cards or is this just for surfpin.