I have huge sympathies with
@mart1602 as that’s a real crappy situation to be in.
I do think that this is part of the risk taken running second hand kit though. You could spend double/triple the amount on a new machine with full warranty etc, or take your chances with an older machine (which many of us do).
If it was faulty from purchase or within a month or so then you’d be within your rights to go back to the dealer - I’m not sure how far a second hand machinery dealer should be expected to go with warranty claims though (and I have no dog in this fight!). 400 hours is a fair bit of work done in real terms - I’m not sure how many firms would warrant 6 year old machines for that long post sale, whatever industry you were in.