@SkyWhisper - I’ve seen mitre joints in wood similar to what you have described, and in every case it was the work of an amateur. I think the seller most likely did it himself as you suggested.
If that had been the work of a carpenter, he should have been fired. Professional carpenters know how to get those miter joints just right.
It takes the right tools, the right measurements, and a good deal of patience. I’ll bet you anything he didn’t use a compound miter saw to cut those angles and just used a regular household saw.