This is a little late, but I had to answer because I know the answer because we just went through this too!
So, the guy was not trying to cheat you out of money...that is the new rule with all upgrades...that all phones will be required to at least take on the $10 data plan (unless you have just a basic, run of the mill, no bells and whistles phone- which very few people have that anymore). The $10 typically gets you unlimited texting and a limited amount of internet...up to the $30 plan which is unlimited of both.
Now, e-bay works to buy phones but make sure to add AT&T in the search so you know the phone is meant for them...but, when you add your SIM card, it will not let you activate it...it will send you to a customer service rep because in their computer system, each type of phone (smart phone, regular phone) and what is on their computer system much match to allow the phone to be activated correctly.
Here is how I know this...I have Verizon and we decided we wanted smart phones but wanted to keep with our cheap $10 texting/internet plan and so came up with the bright idea to just get our phones on e-bay. When they came (and make sure you read carefully on the page to make sure the phone is exactly what you are wanting to get...had to send 2 back with false advertising...only go with "top sellers"- and even then, read carefully), I tried to call to activate and sure enough, it told me that it could not be done and sent me to a customer service rep who proceeded to add the $30 text/internet to both our phones (because we were going to use the internet lots- we went with the $30 plan)...it stunk! It is just yet another way the cell phone companies can gouge us...bad thing is-it's ALL OF THEM doing it...so you can't just skip out on it by switching companies. If your phone has the capability of going online, you will have to have at least the $10 plan (I have a feeling they are dong this because so many people probably accidentally get online- whether on purpose or butt dialing-and then they have to deal with angry, irate people calling asking what the charges are from the internet downloads...and this new procedure will stop these calls for the most part.) Anyway, hope that helps!