The Honda 400 Falcon is sold in S. America. (made in Brazil) Other bikes may be for sale as well. But Brazil is expensive (generally) Mostly 125's and 250's. Big bikes (over 400cc) are very expensive in general.

Word is Suzuki have an assembly plant in Colombia and produce DR650's and
Vstroms there. I have no further details ... but this info comes from a local. Check at Suzuki dealers to see what they've got for sale.

Bikes in S. America are generally about DOUBLE the cost as in the USA. So a NEW DR650 that costs about $6000 USD in the USA, will cost $12,000 USD in S. America. (mas o menos) Good news is you can legally resell it when your trip is done, but can never "import" it to USA.
(you can ride it round as a tourist ... but not register it permanently)

Buying a bike from another traveler is great if you can get the title in your name and work out details with transferring the TVIP. Most borders want to see an ORIGINAL title in your name. Things like Bill of Sale or notarized notes generally will not work. But by far the best value if you can do it.

Or you could buy a nice bike in USA and ride South. You can ride from USA border to Panama in two weeks easily ... but better to take two months!

Tell Tale signs are good to know ...if you have experience and know how to read them. But even then, it's not 100% right everytime. I've bought and sold 50 bikes in the last 20 years ...mostly good luck. So experience counts ...

Looking for tell tales is hard to explain in writing. But the obvious stuff is:
1. Look under bike. Are the lower frame rails bashed and dented in? Is the
shock linkage pieces dinged up? Is the underside of the swingarm gouged?
Bash plate sand blasted?

2. Check the steering stops. Look for rust or bent stops from hard contact (indicates possible crash) .

3. Big dents in rims (indicates bike ridden HARD and FAST)

4. How is the chain and sprockets. Poor chain and sprocket condition can reflect on how the owner cared for the rest of the bike generally.

5. Look at the oil. clean or dirty? Bad smell to it means it got HOT! (not good)

6. Have the electrics been bodged into? If so ... be careful.

7. Engine Sound: This one is hard for the inexperienced ... start engine, and listen carefully. Let it get HOT. How does it sound? Do a meditation and a Zen prayer ... maybe some wisdom will come your way?

Some bikes may click and clack and actually be OK. Normal. Others, those clacks may not be good. It's hard if you don't know the specific model. Expert help is good on this one.

8. Ride the bike. All the gears there? Does it go down the road straight? Weave at speed? Turn left and right evenly, neutral in corners? Brakes strong with no pulsing? does it accelerate OK? Idle smooth? Start when HOT?
A 10 minute ride can judge most all the above.