You've already bought the tractor, you need to get it working to sell it. But once it's working, in general a Kubota should need only the most minimal maintenance, I'd give it another go before I gave up. Presumably you actually have a use for it that means you bought it originally? Once you've seen everything once, then you've learned all you need to know. The learning curve is just steep until you've seen everything once.
We can probably work you through what you need, but just slowly.
So, the theory most people look to have is that you have a fuel blockage. The way we know that is that it starts (so it must have fuel), but after a while it stops. That would often mean that it's got a bit of fuel flow, so it fills the filter and the line. But once it's running it's using fuel faster than it's coming out of the tank, so after a while it stops when the lines run dry. It won't start immediately, but if you leave it for a while some more fuel trickles out of the tank and fills the lines again.
If that's the theory, then you need:
a) a way to check if that's true before you waste time on it, and
b) a way to fix it if that is actually the problem.
The way to check it is to see if you actually have fuel flow. You can do this (or have your mechanic do it) by disconnecting a fuel line and putting the end in a bucket, and turning the key to the on position (not the run). The fuel pump will pump fuel into the bucket. You want to see a good enough flow - I don't know exactly how much, but someone on here will be able to say how much fuel you should see in 20 seconds or so. If it's not coming out very fast, then that's the indicator you have a blockage. You want to use a clean bucket so you can put the fuel back in the tank once you've done the test.
If it is a flow problem, then you follow the fuel line back to the tank. As others have said, it's most likely algae growing in your diesel, or something fallen in the tank that is blocking the outlet. Given your level of experience, probably you'll want your mechanic to drain the tank, clean it, change the filters, put algaecide in to kill any algae, and get it running again. In concept you can do this yourself - it's not hard, you just need step by step instructions. In reality it doesn't feel like you have the confidence to take it on right now, so probably better to have someone else do it.
I hear what you say about nothing working the way the manuals and YouTube show. That might mean you don't have the right manual - generally Kubota workshop manuals are pretty good, if you have the one that matches your tractor the instructions should be relatively clear. Having said that, they do assume a fair amount of mechanical knowledge, and so it may be like asking someone who doesn't cook a lot to follow a recipe - there can be quite a few assumed steps that they just don't realise they had to do.
