I looked at the eBay item. That price ($58 + $14 shipping) is WAY too low for any kind of a quality rotab. No specifications are given for accuracy, squareness, center hole size/taper, etc. You're buying a pig in a poke. It MAY be worthwhile, but I'd be very hesitant to get one of these myself. You generally get what you pay for.
As an alternative, you might just want to look at Little Machine Shop's offerings. Their lowest price 4" rotab is #1810, $290.
http://www.littlemachineshop.com/products/product_view.php?ProductID=1810&category=
Note that the gear ratio for this table is 72:1, twice the 36:1 ratio of the eBay item. Also note they give full specs for rotational accuracy, flatness, parallelism, size, etc.
Regarding mounting, note the grooves visible in the top and left side face. You'd use toe clips to mount the rotab to the Shirline's table. LMS offers mounting hardware kits (link on the right of the page), but they probably wouldn't fit a Shirline. You could easily fabricate your own hold-down clips.
You can click on the "Other Rotary Tables" or "Tailstock and Dividing Plate Set" links on the right.
I've bought lots of goodies from LMS, and they're a quality outfit, very helpful, and stand behind their products.
Best wishes, whatever you end up buying.