No disadvantage, just ask the seller to make sure the unit is unlocked and that the ECU is the same as yours. If you want it new, you may want to purchase from TSP directly as they give the tune for free with the ktuner purchase.
Yeah I just bought a used one for $350 with no TSP and no issues! Uploaded the tune last night with no problem. Just make sure it's a verified seller (anyone with a decent post history on here should be fine, watch out for new accounts with no posts) and obviously do PayPal goods and services to get purchase protection.
One surefire way to prove that they are actually selling a Ktuner and it is unlocked is to request a timestamp in each picture - just a little piece of paper or post it note with username, date and time.
If you find someone selling their unit with the TSP tune, just know that you'll likely have to convert it to work with your car's ECU software. I think its $15 from TSP.