If i bought an Unlocked AT&T phone off of ebay and put my current AT&T sim card in it, does it still work?

So long as the phone is AT&T branded then you won't have any problems. If the phone is non-branded and is just unlocked and capable of being used on AT&T's network you will be able to use the phone for voice calls and text (so long as it has the 850/ 1900 mhz frequencies) but won't be able to use other data services.