Unlocking on eBay?

[anon(7822191)]

So, I need my phone unlocked before I move out of the country. It's a 1520. I called att; I have to cancel my contract early for them to initiate/ give me an unlock code. I'm wondering, though, what makes their unlock code better from sellers on websites and eBay who sell codes for 10-20 bucks? Are these people legitimate? I'd rather not have to cancel early and pay all at once- would be easier for me to stagger the remaining payments whole abroad and not renew my contract. Thanks!