In short, a grey market tractor is one that was produced for another market, other than the us, but was then imported to the us. The big differences will be inthe safety features that the us imposes on domestic models. ( pto shields, rops, etc )
Regional manufacturing differences will also prevail. For instance, the yanmar grey market tractors ( with a 4 digit number, as opposed to the 3 digit us models ) Had a narrow stance, and many had an ag style tire suited for use in rice paddies. Also, some grey model tractors will have a foriegn language manual, and stickers, and there may be some difficulty in obtaining service and or parts, althought, as we have seen on this site, parts are readilly available for many grey's by dealers here on the list, and as long as you are handy with a wrench, most are willing to offer much phone or email tech supt. I have a yanmar 1700, and love it.
For more info, contact one of the grey market dealers or parts suppliers.. The supplier I use is Len schaffer... ( len12 i think )
Soundguy