Originally Posted by Tony LEE

I'm in BA too - just sorting out the gear and the vehicle after it being sitting for 7 months.

Poder is a permission by the previous owner in whose name the TIP and vehicle title is, giving you permission to drive it. It is signed by the previous owner (whose bona fides were checked by the "lawyer"), not signed by you, and identifies the vehicle by type, registration and VIN as shown on the paperwork issued back in the licensing country and identifies you by name and passport number. Also gives any restrictions on allowable use.
It may work and then again, it may not.

Now, this is not to imply that you can't rock up to the border post, get your passport stamped out (make sure they DO stamp it. Another story), front the next counter of the Aduana, hand over the old TIP and he sometimes DOES just shove it under the counter and wave you through (which will likely result in an uncancelled TIP that will show up later). Has happened to me twice at very busy border posts down south. He may also accept the bill of sale plus original papers.
HOWEVER, how do you get into the next country? You may have no acceptable proof that you do own the vehicle unless they do accept the bill of sale, which they might, in conjunction with the original paperwork. Insurance might also be a problem since they like all the paperwork to match up.

Lots of mights and maybes, but you did raise your concerns so ...

Obviously the idea is to cut off any international trade in stolen vehicles.

Maybe we are alarming you unnecessarily because obviously a lot of the vehicle sales down here are not even done with proper bills of sale and I guess they manage to travel around OK. Same with transfers in Uruguay. I know of vehicles that have sat there beyond the 12 months yet then see the new owner driving around down south so obviously there are plenty of holes to get through. A lot of the vehicles aren't worth a great amount so losing it may not be a problem, but some of them are multi-tens of thousands of dollars so walking away is not a nice option Xfiltrate - a long-time member here has been a strong advocate of caution when it comes to buying and selling vehicles and he plainly says it is illegal unless some fairly complex and expensive legalities are completed. Obviously he is correct, but in the real world things sometimes fall through cracks so buying and selling goes on with a high chance of success.

Recent reports claim that two vehicles have been confiscated at two different Argentina border crossings within a couple of months of each other so if they are true - and every reason to believe them - a successful sale is not a given.

Purely as an aside, A couple of my RVs are owned by a Montana Limited Liability Company - which allows me to easily own vehicles in the US, but when we take them over a border, we also need a Power of Attorney whereby the directors of the company - namely me, give the driver - namely me, permission to take the vehicle over the border. To back that up, I need the LLC documentation and personal documentation.
Mexico Aduana insisted on the Poder and I had one printed up with all the relevant details but without having had it notarised in the US, but that was good enough. For down here, I did get the document properly notarised in the USA but shouldn't need it because I changed the name of the LLC to be the same as our jount names so the title and registration have our names on them with just the "LLC" on the end being a bit of a clue. We'll see.