There are lots of threads in this forum on the VN rebuilts. The 'YNM' brand comes from I think the largest reconditioner/importer/wholesaler.
I don't think we've seen this 'Titan' retailer mentioned here before. Looks to me just somebody with good website-making skills but not much experience with Yanmars - he says they test the magneto and test that the carburetion is working properly. I'm not sure I would want that guy doing any warranty work. If they've been around long enough to even have a shop to do warranty work.
These are simply a gamble, many people have excellent luck since these Yanmars are pretty much indestructible, and a few customers get a tractor where the reconditioner didn't repair a problem despite the museum-quality paint job.
Here's a thread I linked to yesterday. In summary the VN rebuilt was assembled with a fuel pump that had cracks in the cast iron body and a big chunk missing. (see the next to last photo in post #1). No way would a conscientious mechanic assemble a 'new reconditioned' tractor using junk parts like that. This experience points out that these 'rebuilders' have NO access to genuine Yanmar parts. If they need something expensive it has to be salvaged off another junker tractor.
I would definitely pay extra for an extended warranty (from the importer, not from some guy who bought one trailer load of rebuilts to re-sell) and only buy if the transport to send the tractor back for warranty work wasn't too expensive.
You pay your money and take your chances ... Most people win the gamble ...