You can never sell them.

You may only trade in lower valued ones when purchasing higher valued ones for the same country, and are in that situation given credit equal to their cost. Only 1 exchange per purchase.

You cannot sell bonds.

However, when you buy a new bond, you can trade in a bond when you buy a higher one, i.e. you could upgrade your "5" bond to a "6" bond by paying 4,000,000. In other words: you are not stuck with the same bonds during the entire game, but you can only increase your stakes in a country, not lower them.

Nope. They're yours.

You may exchange them for more expensive ones (of the same country) through the course of the game. But once you buy into a country, you've bought into that country for the duration of the game.

I'm not sure but looking at the replys i would say:

you can't sell them

BUT

you can trade them in when you buy more expensive ones of the same country....

Thank you all. That's what I wanted to know.

Just to clarify a little further, you can only trade in one bond when upgrading to one of a higher value.

You may not combine two or more bonds and trade them in to acquire a single bond of higher value.