I have not purchased a tractor from Alibaba but I have done serious investigation into buying some implements through there.
Aside from brokering the actual sale of whatever, you will also have to arrange everything else to get it to your door. The seller will usually deliver to one or two of the closest shipping ports for free but after that it's your baby.
That means you need to arrange:
1) A carrier (i.e. a boat) to bring it from that port to a port in the U.S. As has been pointed out already, you're probably not looking to buy an entire containers worth of tractors so you need a shipper who will take LCL (Less-than Container Load) freight. There are two ways to go about this. One is to find a "Freight Forwarder" which in simple terms is someone who pays for shipping an entire container from port to port & then resells space in the container till they fill it up. The other is finding a shipper that will handle LCL directly, which in most cases means it will be lashed to the open deck & exposed to the weather in whatever packing the seller sent it in.
2) You'll need someone to pick it up from the seller at the port & to make sure it gets on the ship.
3) You need someone to make sure it clears customs going out
If you used a freight forwarder above in Item 1, they will take care of these, otherwise you will need to find & arrange all this through individual handlers & brokers.
4) If you made it this far (and your ship did too) then now you need someone to get it off the ship and take custody of it & get it thru customs. Again a freight forwarder will handle this or you can arrange your own broker
5) Finally out of customs and in the good old US of A - you need someone to pick it up at the port and deliver it to you. (DHL, FedEx Freight, private shipper, etc)
So let's say we clear ALL those hurdles - let's not overlook a couple of other important items.
1) You OWN whatever it is from the time it leaves the sellers hand at the foreign port. So if it gets lost, stolen, damaged, sunk, kidnapped by pirates (ok maybe not pirates) - YOU are out the funds for it . . . unless you buy insurance for it. I'm sure international shipping still has better odds than any game in Vegas but do you really want to take the chance?
2) If you ever have any issues with it, your only support from the seller will be in the form of e-mail or possibly some internet Live Chat and any documents they send you are typically not the top end technical kind.
I know there are a couple folks who have done this with non-powered equipment - the EPA part adds a whole new twist on things to consider - so the process is do-able, but at what cost? Is there really any savings to be had in the long run?
If you're really that interested I can provide you with some of my research links (nothing you can't google on your own but at least I know these folks will answer your questions).
What I determined was that there are plenty of U.S. based importers already selling the implements I am interested in. They offer 1 or 2 year warranties, they have live persons I can speak with if I have a problem, and their prices are not far above what it would cost me to do everything on my own (hey those guys have to have a profit margin, it IS a business after all).
Your mileage may vary & your research might find something mine didn't but my free advice (worth what you paid to get it) would be to steer clear of importing and look for a U.S. based vendor.