Quite a few questions, I'll give you my best info, others may have different experiences.
<font face="Verdana, Arial, Helvetica" size="2">I have a bike registered in Guatemala. I want to drive down to South America, and leave it with someone else to drive around for a while. Will they be able to cross borders with the bike registereed in my name?</font>
I would imagine your friend will have difficulties at borders if the bike is not registered in their name. You might try giving them a letter in Spanish authorizing them to drive the bike, along with a notarized copy of the registration in your name. But, no guarantees that will work at all borders.
In the case of a US registered bike being sold in Panama, for example:
What would the sellers/buyers be planning to do about the registration/ownership documents?
Pay import/registration taxes in the country of sale?
Process the paperwork in the country in which the bike is registered?
Each country has their own rules for these things. Some countries are relatively easy (Bolivia), others completely prohibit importing a used vehicle. And the rules change. For a good discussion on this topic, see the following thread:
Officially, if the country you wish to sell the bike in allows the import, the buyer would have to pay all duties and taxes. They would also have to put it through whatever process exists for registering it locally.
As the seller, your main concern is whether you have a stamp in your passport (most of Central America) which requires you to have the bike with you when you leave the country. If you have all the paperwork demonstrating duties and taxes were paid, etc., you might be able to get out of the country without the bike.
Alternatively, if you have a carnet de passage, you must get appropriate stamps by the customs officials in the country which you are selling it in proving you have paid all duties and taxes on the vehicle sale. Otherwise, you will forfeit your carnet deposit.
It has been said that with a willing buyer and seller all things are possible. But, my response is that I wouldn't want to end up in a jail in South America for running afoul of the customs and vehicle import laws.
'One world, two wheels'