The ad does say $450 or best offer.
This implies the owner would like to get the full price, but that he is willing to take less.
This works to the buyer's advantage.
At this point, the "buyer" can low-ball the seller, and possibly piss him off, or ask the seller what his best price is and work from that point. .....Another option is to offer what you are comfortable paying.
Seeing and running the tractor will provide a better idea of what it may be worth to you, than just looking at a picture!
If you just want a decent running tractor that is not 100% original, your offering price will be at one level.
If you plan on "blowing it apart" to do a 100% restoration to showroom perfection, you will end up investing more $$ than you will ever get back when selling it. .....If this is your plan, look for a less-expensive tractor, as you be buying many new parts.