estate licensing? Do the owners of the websites have to be licensed real estate brokers?

No, but some MLS organizations have prevented use of MLS material on those sites without proper authorization. You need to check with the local boards and your state real estate commission for the rules and regs for whatever area you are interested in.

I don't think they need so. The give information and when a transaction is done, that will fulfill all the laws in force including transfer, registration and taxes.