You can buy EU CO2 certificates and resell them for a profit which makes emitting CO2 uneconomical because of all of the middlemen who are taking their cut. If you never sell them below the purchase price they won't result in emissions. It's a win win situation.

it's true, offsets can't prevent purchasers from adding to their emissions, if they choose to act in contradiction to the spirit of purchasing offsets in the first place (mitigating the emissions which cannot be immediately reduced for whatever reason to buy time for when they can be reduced).

however, they can fund projects to help people with less money to reduce their emissions by an equal amount, generating a large value in beneficial externalities as a result.

if i purchase an offset and the proceeds go to a hydropower project in india, there's a lot more going on there than the compensation for one ton of emissions alone.