Comparing Paypal with Paisapay is like comparing a cat with a dog. Paypal is more like an online Bank where you can park your funds(without interest), transfer them through internet from urs to someone else's account. Also it has been integrated in eBay but functions separately.
Paisapay is a service offered by eBay.in . It's like you are using turbolister or selling manager, It will only deduct 5% or so from what buyer is paying and will transfer the fund to sellers account once buyer confirms receipt. You cant store your funds there nor can you use that money for purchases unlike paypal.
Paypal is a big company and is considered western union of internet. Its business for year ending in July 2009 was over $669.3 million(including profits of bill me later).. (Also to add even skype is owned by eBay)
So building something like Paypal in India in not only difficult for eBay but also is not worth as paypal itself can be used in India.