Invalid merchant set up?

I am using Version 1.3.7 and a customer has told me that when they try to check out they get a message saying invalid merchant set up. And idea how I can fix this problem? Thanks.

And idea how I can fix this problem?
You have provided no information as to what you have setup, or where one can see this???
Correct the setup and do your own testing instead of waiting for a customer to tell you that there is an issue.

Well it was working fine yesterday. I just thought that maybe it was an easy fix. I will get my people to look into it.

I just thought that maybe it was an easy fix.
You still have not elaborated on or provided any information as to what "merchant" is not setup??? Or how you know it was fine yesterday.
Am I correct in thinking that this is a payment gateway issue?
Gotta know what it is before any fix can be suggested

Well I did not get much info from the customer. It was a gateway issue with PayPal. I just tried checking out and it worked fine. This is a guess but I believe it may have been because he wanted 50 pieces of one product but there was only 25 in stock. Would that cause that problem? Also when I test my cart how do I test it without having to go through with the transaction and having to pay PayPal fees and such? As you may be able to tell I am new to this and would like to learn it. Thanks and any info is appreciated.

Also when I test my cart how do I test it without having to go through with the transaction and having to pay PayPal fees and such?
You must use another PP account different from thr one the cart is setup with. Just process tests as normal and after refund them.
Extra step, but one has to know what works & what does not
I genrally create a product for testing that is $0.10 and do not worry about refunding and PP does not assess any fees for that amount