Can a reserve officer hire one of the soldiers in his unit to work temporarily at his business? Does this rule differ for active duty member?

See AR 600-20, para 4-14, c (1).

“ c. Certain types of personal relationships between officers and enlisted Soldiers, or NCOs and junior enlisted Soldiers, are prohibited. Prohibited relationships include the following:
(1) Ongoing business relationships between officers and enlisted personnel, or NCOs and junior enlisted Soldiers. This prohibition does not apply to landlord/tenant relationships or to one-time transactions such as the sale of an automobile or house, but does apply to borrowing or lending money, commercial solicitation, and any other type of ongoing financial or business relationship. Business relationships between NCOs and junior enlisted Soldiers that exist at the time this policy becomes effective and that were authorized under previously existing rules and regulations, are exempt provided the individuals are not in the same unit or chain of command and the relationship does not meet the criteria listed in paragraph 4–14b(1 through 5). In the case of ARNG or U.S. Army Reserve personnel, this prohibition does not apply to relationships that exist due to their civilian occupation or employment.”

On my deployment this past year our ARNG partner unit had a ARNG MAJ who did the financial records for his subordinate SGT's business in the civilian world. It was wild to me but apparently it's worked for years with them. I'm not sure of the legality of the situation but it happens.

Im AD and i regularly do one offs for people on base. Rank never comes into the equation when it’s a matter of “Hey Chris I saw your facebook ad, can you build me a computer?”

Also I don’t advertise that I’m in the military on my listing, just the area I’m willing to deliver in.