Buying courses

I have never bought online courses of anyone before.. I am planning to buy Vishal bhattad's exam oriented IDT and Nitin goel's advanced accounts revision.. however I don't know from where to buy these courses.. there are several sites offering these courses and I don't know who to trust.. are there any discount codes available? can someone guide me through it?

Also I can't even find niting goel's lectures courses anywhere.. helpp