Honestly, I don't know if that eBay search item would work or not. I'm used to traditional microscopes and with using a mechanical stage that has the knobs to make scanning a slide easy to control. As a test for coccidiosis, they're in large enough numbers to be fairly easy to test for and it doesn't take much slide-scanning to make a determination.
However, with roundworm and threadworm eggs, they're often a lot more difficult to find. There might only be one or two on an entire slide. That takes considerable scanning--about 15 passes over the coverslip from end to end at 100x total mag. The product shown in the eBay link wouldn't be as good as using a kid's microscope for that. The other thing that you can't be sure of until you have the product in hand is the resolution. It's easy to get high magnification but still have an image that fuzzy enough that you're not sure what it is.
For doing closer work to begin differentiating bacteria and protozoa, you're going to need a lot more power--up to a 1000x oil and that means a real microscope. They're easy enough to come by, especially if you don't mind monocular. However, the newer ones aren't going to withstand the abuse that the older ones will. That has to do with the fine/coarse adjustment gearing.
Pidgey
