Which web service helps to compare prices of same product from multiple online stores?
You should try Compare India
Storewala.com is also a good site.. I use it often to compare prices in india
Do they give MRP or maximum retail prices?
findthebest.com is one of the comparison website you can use it.
you could check at some of the articles from yours trully Makeuseof;
There's a firefox addon called PriceBlink that I like. Basically, when you are on a web page looking at a product, it pops up telling you where you can find that product for cheapest.
InvisibleHand and CamelCamelCamel are some of the best I found for amazon.com and many products in the US.
and to some extend Compare India.
It seems you are an Indian , just like me :) ... well, there are few websites that can help you with this .
3. eBay IN
5. India Times Shopping
This 5 will work perfectly :)
There are plenty (e.g. Google Shopping, InvisibleHand), but which one(s) you should use depends to a great extent on where you are. Which country are you in?