It looks like a good deal. These motors will seem like they turn slower compaired to your Poly-Sci ones for one reason these have twice the amout of encoder counts on them. Each pulse moves 1/2 the distance of your other servos.
The most important thing to find out is the peak Amps of the ebay servos. If they can handle the 20 amps of your Poly-Sci ones then the samp power supply will be fine.
Thanks
Jeff Davis (HomeCNC)
http://www.homecnc.info
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)