There's a saying in dressage. You make the trot; you buy the canter and walk. In other words, since the trot simply shows a lack of impulsion, I wouldn't worry a bit about that. From what I see, I have no doubt there's more trot in there-he's just not being asked for it as per the rider's preferences and training requirements. He has 3 very decent gaits, and the canter shows reach. He looks to have a good build and a lovely thick neck at the shoulders. He is not 'false round" or BTV. Its difficult finding all these qualities in one horse.
This horse has a looseness in his movement that I haven't often seen in TBs - I honestly thought he was a warmblood x TB. Most of all, he looks very rideable and trainable. doesn't look spooky, the rider carrying/switching crop doesn't faze him, he pops up the little bank without a thought, he seems quite broke. I think he's quite a find - if the price is right and he vets and you like him.