Ebay and Paypal
I have to respectfully disagree with you guys saying Ebay is full of scammers. I've both sold and bought on Ebay, probably at least 18,000 worth purchased since I started my account. I've probably sold at least 7,000 worth of stuff as well...
When it comes to protecting yourself, it's easy. You need to only sell it to someone with a verified Paypal address. Also, ensure that you select signature required on delivery. Then, there is proof that the person received your product.
I would say tell the person that you will ship them the SIM card, and then when they receive it and leave you Feedback on Ebay, you will immediately send them the password to the account. The only possibility of you getting screwed besides that, is if FedEx or USPS loses your package. This is not common but can rarely happen around holidays.(This has happened to me once)
The good thing about this particular type of sale, is that with this product, if it gets lost, who cares, because you can just refund the buyer, and go to ATT and ask for a new SIM card to be made. Then you still control the account, and can get a new SIM card at the low price of $10-15.
Ebay tries to protect the Buyer usually, but PayPal, although they are owned by Ebay, does side with the seller almost always, as long as you can PROVE you shipped the item(tracking slip), to the proper address and the customer received it(Sign on Delivery), then PayPal will make sure you get the money you are owed.
I don't mean to make fun of anyone, but the scams that exist on the internet are as easy to avoid as getting viruses from sketchy websites, or buying a used car that's obviously not worth the money, call Ebay or PayPal and talk with them about your concerns and they will let you know all the options that are available to you to ensure you are protected as both a buyer and a seller. They are a company like any other, that wants repeat business.
I'm not saying that you need to trust everyone on the internet, skepticism is great, but just deal with honest people/companies and you will be fine.
I'm sorry, I'm ranting now haha I hope my information was helpful to you.
GL with your sale or whatever you decide to do!
Edit: One more thing. Ensure that you have a VERY thorough description on your Ebay listing, and that it states that you will not be responsible for their inability to update the account information and lose the account etc. Be Specific!!! Especially when selling this type of product, where some people may not fully understand it.