I have a website that I'm trying to hook paypal up to with a php script. php builds the query string and then submits to https://www.paypal.com/cgi-bin/webscr with the query string attached. I can verify that this works fine. I haven't processed a payment but it directs to PayPal with the correct quantities and prices, etc.
However when I change to https://www.sandbox.paypal.com/cgi-bin/webscr I get an error when I try to process that says there was a problem with the merchants PayPal account.
I have created several test accounts on PayPal sandbox but from what I can tell looking around it seems I need another merchant account in the sandbox? If so, I don't see how to do this. I'm guessing I need to change the "?business=mycode" portion of the query string but I don't know how to figure out what to change it to...
Anyway I looked around a while and can't find the answer, can anyone help or direct me to another post that answers this (I looked but couldn't find it, or at least didn't understand it if I did)?
When changing back from live to sandbox, all you should need to change if using a non hosted button is the URL to reflect the environment that you are trying to test with and the value of the business variable. This can be set to either your email address of the actual test or live seller account or the merchant id. If you have not already created an account for testing. You can do so by going to PayPal's Developer Site, and creating a developer account. Then once you have done this, you can log into the developer account and go to the Applications tab. Once on this page, click the link for sandbox accounts on the left, and create a preconfigured test seller account. This will generate an email address linked to a test account. This will be the email address that you will need to use. If this doesn't work, can you provide the string that you are submitting over to PayPal or the button code that you are using and I will take a look at it.
There is error in API key for sandbox testing in Paypal... It is due to corrupt key generation which is not properly decoded on sandbox server when you send a purchase request.
I tried using following sandbox public credentials and it works fine for me
username: sdk-three_api1.sdk.com
password: QFZCWN5HZM8VBG7Q
key: A-IzJhZZjhg29XQ2qnhapuwxIDzyAZQ92FRP5dqBzVesOkzbdUONzmOU
however I tried several times using the sandbox merchant account that i have created myself.... but was unable to ACK success.
来源：https://stackoverflow.com/questions/16174859/paypal-sandbox-merchant-account-error