I don't have a complete answer but the response I received from BuySafe support is below.
The answer is incomplete because two problems were solved and a new one arose. I was now able to successfully authenticate the seal. I was able to see the button in the shopping cart. But when I went to checkout, I got a blank screen. I'll edit this post or reply if I get the rest of the solution. This solution should also work for OsCommerce and other similar carts.

The 3 files can be downloaed from
http://ifscal.net/nusoapPatch/nusoapPatch.zip

Hello,

Here's an explanation of why you are seeing the error:

buySAFE uses the NuSOAP client, which declares 2 classes: 'soap_server'
and 'soapclient'. PHP5 comes with an optional SOAP library (compile with
'--with-soap'), which declares 2 classes: 'SoapServer' and 'SoapClient'.
On machines that have this new PHP library installed, then there exists
a namespace collision. We had to rename references to soapclient to
nusoapclient in our implementation.

To fix this, you will need to download and unzip the file that I have
attached to this message:

Save buysafe.php into \includes\modules\buySAFE\buysafe.php

Save the other two files into both

\includes\classes
\admin\includes\classes