We want to integrate our ASP.Net webapplicaiton to PayPal. Currently we are in teh process of developing the application so we are using PayPal sandbox. When user clicks on checkout button we pass the control to paypal sandbox, where the items along with their price is displayed. Now suppose user pays for the items, now control should come back to the main application along with the information whether payment was successful or not and also the transaction id (which we need to put in our database). Please let me know how we can do this. Also, do we need a public ip so that sandbox can send the control back to the main application. Currently the application is under development so not deplyed on any servers with external ips.

Paypal's SDK kit for ASP.Net may be a good start. There are instructions, documentation and sample code available on that site. 
iHaathi
This post is given AS-IS. No Warranties.

Check Rick's article http://www.west-wind.com/presentations/PayPalIntegration/PayPalIntegration.asp
Thanks
Mark post(s) as "Answer" that helped you

Electronic Screw
Website||Blog||Dub@i.net

well go to paypal web site and login with paypal's account username , you will see there two urls one if successful and other for unsuccessful. so you will be redirected to that page.
you can find how to get transaction id in pdf file available there.
if still you need code of getting id you can post i will write code here.
http://www.munishbhatia.com

Hello,
These might be helpful
 

 

  
http://pdn.paypal.com/commercestarterkit/default.htm
 
 

  
https://developer.paypal.com/
 
 

  
http://www.west-wind.com/presentations/PayPalIntegration/PayPalIntegration.asp
 
 

  


 
 

  
http://www.codeguru.com/csharp/.net/net_asp/miscellaneous/article.php/c13851/
 
 
Mehedi Hasan


Mark as answer if the post meets your requirement!