Usually, just their ID such as a drivers license. 

Can somebody please respond or point me to a post that will help me learn how to do proper due diligence on a wholesaler before doing a deal with him/her?

It cost thousands of dollars to get started and maintain your license as well as the recurring continuing education you must put in every 2 years (at least in AZ).If you're looking to get into wholesaling or buy and holds, it probably won't benefit you as much as if you were a flipper.

Being a licensed agent is expensive.

I recently got my residential contractors license in Florida and formed an LLC as the only managing member.

I think it would be very difficult to enforce the property delivered vacant as the tenant needs to be served proper notice. 

I've renovated SFHs before but I am not licensed.

I am a licensed financial planner in the San Francisco area with 16 years experience.

For example, I help people get turnkeys all the time and I'm a licensed real estate agent, but I don't put myself formally on the paperwork as the buyer's agent.

I worked with a real estate data research firm, REIS out of college and I obtained my NY state real estate salesperson license to do a little renting.