The size looks good. 51" x 20.75" x 3/4" is good all the way around, and well within the parameters of the general sizing I recommend for most people when they look for a new kite-skim. I would agree with your expert on the strength to dollar ratio. An EPS core, vacuum bagged, and s-glass on the top and bottom is the same type of construction that is used on my Phase 5. However, since the board is so inexpensive it must be a China knock off board. I think for $99 it probably worth a gamble. Heck, you can blow $99 in one round of drinks.
Since its designed mainly for surf it might be a bit aggressive on the nose and tail rocker, and not work as well in lesser wind conditions. I would like to see a detail shot of the rails and rocker measurements. However, again it would be hard to go completely wrong at that price. Worse case scenario is you use it for your slider board!