Is it legal in the United States to hire a test pilot to conduct the first flight(s) of a homebuilt ultralight? This seems like a bit of a grey area- FAR 103.1(b) states that an ultralight aircraft is a vehicle that “is used or intended to be used for recreational or sport purposes only” but I don’t see any reason that a pilot could not be paid to evaluate the flight characteristics of the aircraft.
The FAA highly encourages you to find or hire a test pilot for your E-AB.
This advisory circular (AC) provides information and guidance on the Additional Pilot Program (APP) for flight testing experimental aircraft. The APP was developed to improve safety by enhancing Builder/Owner Pilot (BP) skills and mitigate risks associated with Phase I flight testing of aircraft built from commercially produced kits through the use of a qualified additional pilot and powerplant testing. The APP is an optional program which provides another pathway to conducting Phase I flight testing. The traditional option for a pilot to test their aircraft solo during Phase I is not covered or affected by this AC, and remains an option for those who choose to do so in accordance with their aircraft’s operating limitations.
16. COMPENSATION. Compensation for the services of a QP or an OP may only be accepted or provided in accordance with the privileges and limitations specified by certificates held by the individuals.
This means in order to accept compensation, the test pilot must have at a minimum, a commercial license, appropriate medical, and be qualified to fly the aircraft. I'm not entirely sure if recent court rulings require a LODA though, you may want to check with your local FSDO.