I'm not sure I understand your logic. T-Mobile isn't offering 50% off plus a trade-in. It was offering up to 50% off with your trade-in. Assuming you were paying on an S8, you have to pay the balance off on the S8 first, then trade it in. You don't get to turn in the S8 for $350 while you you still owe over $300 on it. Seems like you would need to account for paying off the phone before trade-in.
Maybe I'm just misreading your post. I'm selling my S8+ on ebay. It's already up to $450 with a day to go. Selling it outright is definitely a better way to get top dollar. eBay has a promo out there where select customers can get guaranteed sales value if they list it how ebay wants them to list it. I was guaranteed $417 from ebay, so I jumped on it. If it had sold for less, I would have gotten the difference from ebay.
Here's the details of the deal:
eBay's Price Guarantee
I'm also trading in my old S7 Edge for $300. Didn't think I could get that much for it on ebay and Samsung gave me the amount off the order instead of through bill credits. When you tack on the free money from using ebates, and the reduced tax burden for taking $300 off the cost of the phone in advance, I come out just as well without dealing with bill credits from T-Mobile.
For anyone looking to cash in their phone, they may want to see what they can get from ebay for it first (if they even offer you the deal).