Does anyone knows how i can get buyer's reviews from a Product in a .Net Project from eBay API (soap , rest..). Is there a way to do it with .net API? Or must i build some custom crawler.
You need "FindReviewsAndGuides". You can search by product, by user, or by both:
http://apidocs.ebay.com/Shopping/CallRef/FindReviewsAndGuides.html
You can call the SOAP API with .NET. There is a quick-start guide here:
http://developer.ebay.com/DevZone/xml/docs/HowTo/FirstCall/MakingCallCSharp.html