First of all the device doesn't have LTE, so that doesn't have anything to do with it. What the ebay ad is referring to is that T-Mobile is upgrading some of its network to use AT&T devices such as that one on 3G. However, there is no guarantee they will upgrade their whole network for that in 2013. Thus I wouldn't count on that.
If AT&T has good coverage in your area but you don't want to pay AT&T prices, consider Straight Talk $45 plan and get the SIM card for an AT&T compatible phone.