With states taking a more active role, you may also have state accredited offsets (such as the California Air Resource Board’s ARB Accredited Offsets). California is also entering an international agreement to trade emissions offsets with Quebec. To add to the complexity, you have primary and secondary markets, where offsets can be traded back and forth (like futures contracts). Sometimes to boost credibility and assure that the projects are actually reducing emissions, they can be double verified or pass other tests, which results in higher costs.
Because of all these variables, the main rule of thumb is to buy offsets from a reputable organization that is following an offset protocol or standard. This ensures they are developing their offset projects along strict guidelines that are verifiable and accepted as standard practices. For example, click here to see that the Climate Action Reserve has protocols (rule books) for different types of projects (this is similar to how IPMVP has guidelines for different energy projects). You can also detect who the reputable players are by their client list and who is putting trust in the idea that their carbon offsets are bullet-proof to scrutiny.
There are many other site-specific factors to consider to optimize your offset purchases. For example, in some areas, you can only offset up to 8% of your footprint. If this topic is important to you, learn what rules are going to apply to your location or facility type.