My question is simple. I know there are Grade-A & Grade-B for blank CDR.
Is there any way to differentiate between these 2 grades ?
or should we just rely on what the seller tell us ?

I've never heard of a grade a or b for blanks but that doesn't mean there isn't such a thing. The best way to judge the quality of a blank is by where it is manufactured. Japan being the best place currently. Mitsui, Mitsubishi, and Taiyo Yuden are all Japanese manufacturers that are very well respected for their quality of materials and production standards. Their discs normally have a blue or green tint on the underside. Taiwanese made discs are generally frowned upon by those that desire quality. These discs normally have a light yellow tint on the underside.
You can get the HQ Japanese discs on the web for around $0.20 - $0.60 a piece(in bulk on a spindle) depending on where you look. I would suggest going with the branded discs as they have a better protective layer on the top and will certainly hold up better in the long-run.
Here (http://www.american-digital.com/prodsite/category.asp?c=51&name=Mitsui) is just one place that sells these HQ discs. And no, I don't work for them, I'm just passing on good info.