Can green tags or TRCs can be used to substitute RECs?

Also, I am having a hard time understanding the concept of how RECs work. Basically, you purchase an REC and the electricity is being used by another party. Is the consumer who purchased the REC the same person who generated that renewable energy? Meaning lets say that I produce renewable energy and decide not to use it. I would sell that energy in return for an REC? Am I making profit?

Also, what does it mean to trade RECs and to retain RECs?

Thanks for your help.