I have thought if doing this before. I don't think ATT really cares. Think about it like this:
ETF: $325
subsidized iPhone 5s $199.99
AT&T monthly service: Approx. $80/mo
If you bought the iphone 5s and kept it though the contract, AT&T would get $199.99 + $80*24 = 2119.99
When you buy the iphone, sell it to someone else (who is locked to AT&T), and pay your ETF, AT&T would get:
$199.99 + $325 + $80*24 = $2444.99.
Technically, AT&T MAY be losing a sale on an iPhone 5s to a particular customer (but probably not). And I doubt they care since your ETF was more than the subsidized cost of the iphone 5s anyway. So, let's just say you ARE causing AT&T to lose out on an iPhone 5s sale. They are still going to make:
$199.99 + $325 + $80*24 - $199.99 = $2245
Which is $125 more than they would have made had you just let your contract end (and the other customer went in and bought a subsidized 5s from the ATT store).
Your net profit is going to be something like:
$750-($199+$325) = $226, and that's neglecting tax and whatever fees AT&T makes you pay when you re-up (and possibly ebay fees depending on how you sell your 5s). So you have to decide if it's worth it. You're probably only going to net around $150ish. I might pay that much just so I don't have to walk into an AT&T store in the first place
In the end, both you and AT&T come out ahead right around $125. So, it's sort of a win-win. Except that you're helping AT&T make a profit and you're also taking all the risk....