Being a "Transaction Processor" in the OmniBazaar/OmniCoin network is similar to "mining" in the bitcoin network. Both serve to process the transactions of other users. Both serve to secure the network. Both involve a reward. And, both involve some work.
In the case of Bitcoin, the "work" is solving extremely complex cryptographic problems using specialized equipment and a LOT of electricity. This system is called "Proof of Work".
In OmniBazaar, the work is providing various services for users of the marketplace, maintaining a good reputation, and referring new users. This system is called "Proof of Participation".
You can learn a