The antenna is a CoCo antenna (Coaxial Collinear Antenna). These antenna are very easy and fairly cheap to make but tuning can be a problem.
A coworker followed these instructions and made a 8 segmented CoCo antenna using cheap RG coax cable cut to specific lengths.
balarad.net
CoCo antennas use 1/2 wave segments (about 5.4 inches for ADSB * the propagation speed of the coax ) . The ebay listing says it is a 4 segment CoCo.
There are people posting about CoCo antenna on FlightAware all the time. Definitely check out the post below:
ads-b-flight-tracking-f21/coaxial-collinear-coco-antenna-hyps-facts-t35941.html
The other choice is to make a cheap Cantenna antenna. These are great antenna and which doesn’t require tuning. You will also probably get better range from an cantenna since they work without tuning.
ads-b-flight-tracking-f21/three-easy-diy-antennas-for-beginners-t20177.html
The FlightAware antenna on Amazon is a great antenna if you want a premade, tuned, and weatherproof antenna.