Maybe I am not just misunderstanding your problem, but how are you going to be able to transfer USDT (ETH) to the seller if you don't have ETH gas for transactions?
Secondly, why not just swap the LTC you already have for ETH? There are so many exchanges that you can use to carry out instant swaps.
When I will get 0.0001 ETH (I probably need even less - at the moment the fee is 0.000033 ETH but it changes every second), I am able to swap my USDT (ETH) to Litecoin. And then Litecoin to Etherum (if needed) to pay for my transaction fee.
I unfortunately have no Litecoin in my wallet I am able to use at the moment (I have some Litecoins in Kraken exchange but I am not able to use them at the moment because of my address proof they do not accepted by Kraken).
