I can confirm what Pecha72 said about Southern African countries. That is exactly how I covered ~8 countries in the region in 2011.
"Buy in a country, and sell in the same country, ok could be do-able, but to really do a RTW-tour this way?"
This won't work at all, or it will require tons of time and effort. At most borders, there aren't people just waiting to buy your bike. Even if you find someone to buy your bike they will want a ridiculous discount because you are a "rich foreigner that needs to get rid of his bike ASAP". On top of that, as a rich foreigner you will probably be paying a substantial premium when you buy so you get screwed on both sides of the transaction.
Some regions are full of bikes and it might be easier to do this because motorbikes are popular, but in many countries motorbikes are not popular at all and no one is looking to buy your bike. It will be a PIA to find and buy a bike, and it will be an even bigger PIA to find someone willing to buy the bike from you. The only way this would be possible is to fly to some big city, rent, tour the country, and fly to another big city in another country. The problem with that is that there are very few countries that have cheap motorcycle rentals. I've looked all over, and there are no more than a dozen countries with <$50/day moto rentals.
I've ridden in Thailand twice and covered most of the country on a motorcycle. When discussing Thailand, it is necessary to distinguish between scooters and "big bikes" - they are not treated the same. As far as I know, it will be impossible for you to buy and register a "big bike"/"regular bigger cc motorcycle" on your Tourist visa and without that paperwork you will not be able to cross into neighboring countries. And if you rent, virtually all rental places require you to leave your passport at the rental place for "big bike" and the bike paperwork is not in your name so you can't cross to other countries anyway. Scooters are a lot easier and no need to leave your passport.... but they are scooters. People correctly say that leaving your passport at the rental place is not a good idea, but this is how every place in Thailand renting "big bikes" does it. (I've heard of people leaving multi-thousand deposits to get around it, though. No one has been able to point me to even one legitimate shop that doesn't require leaving a passport for a "big bike" rental.)