The idea of "protect the rainforest" offsets, and "avoided emissions" offsets in general, is as follows:

1. Party A is planning some carbon-emitting activity, such as cutting down an acre of forest, or operating their diesel bus.

2. Party B is also planning some carbon-emitting activity, such as operating their cement plant.

3. Party B pays party A to halt / avoid their carbon-emitting activity.

Note that nowhere in this scenario is anyone removing any CO2 from the atmosphere; they're just jockeying over who gets to keep emitting.

If you're thinking, "but merely by existing, the rainforest is actively pulling carbon out of the atmosphere", then that's a separate, complicated topic and not what I'm trying to address here. If you prefer, strike out "preserve an acre of forest" and substitute "replace my diesel bus with an electric bus".

> what happens when all of the Amazon offsets have been sold and I still want to emit CO2?

Again, to avoid getting bogged down in complicated analysis of the carbon dynamics of forests, let's focus on the diesel-bus example. Each year, I can pay the bus owner to continue leasing an electric bus, rather than the diesel bus, for that year. I can keep "offsetting" my cement plant forever.