It is my understanding from the numerous comments here on TBN that the NH273 is one of the very best older, square balers out there. It was the No.1 - No.2 baler on my shopping list along with the JD336.
My Dad came across a good condition, 336 at a local farm auction and solved my buying problem..
I think that the key issues to look out for are the same as buying any older, used baler. Stuff can be worn, tired and generally sloppy. Look for excessive wear and slop. Drive-line, plunger head and rails, bale chamber, billhooks, knotters and gears.
When components are excessively worn, it causes glitches with the overall timing of the baler's operation. A missed knot, a string that's not completely cut and 2 bales get tied together. The baler works -- it just doesn't work -- consistently. And will give you a fair amount of frustration!
AKfish