I tried testing out purchasing using Paypal's sandbox.  Maybe this is easier for me because I am in the U.S., but setting up Paypal was not that bad -- after ten minutes of banging my head against the wall -- and once I had my Business and Personal sandbox accounts it was easy to setup things in Symbiostock (thanks Leo and anyone else who worked on the Paypal IPN stuff!).

For those who are just setting up their sites and want to test out purchasing, here is what I had to do, although  because of all the damage to my head the beginning part is kind of fuzzy ...

Note:  This does not include signing up for a Paypal merchant account as I am working with an existing account.

• Sign up with Paypal Developer
  Select the "Log In with Paypal" button and then login using your Paypal Business account.  This seems to do nothing except change the Login button to text with the name on the account, but it creates a Business sandbox account for you automatically.  The next thing to do is create a Personal sandbox account.
  
• Setting up/Managing Sandbox accounts
  From the Paypal Developer page click on the "Applications" button.  On the left edge of the window select the text, "Sandbox accounts."  Click on the "Create Account" button at the top and setup both a Business sandbox account and a Personal sandbox account.  The Business sandbox account will be a stand-in for your business/merchant account and will receive (fake) payment.  The Personal sandbox account will be the test client who will (fake) purchase stuff from your site.
  
  Here are the settings I have for each account:
  
  • Country = (the default setting was my country so I left it that way)
  • Account type = Personal
  • Email address = Use an email different from the email associated with the admin account on your Symbiostock site.  If you already have a test user account on your Symbiostock site you can use the email associated with it.
  • Feel free to set the fake user's first and last name if you want.
  • Bank verified account: Yes (this was the default setting for me and I left it that way)
  • Credit card type:  Visa (I did not play with the other options)
  The other fields I left alone.
  
  Once you create an account you cannot change the email address, so if you need to create an account with a different email address you can (delete the account, if you want and) make a new one!
  
• Integrating Symbiostock
  As admin on your Symbiostock site make your way to the Symbiostock settings (the bee!) and click on the "Settings" tab.  Select the "Sandbox (testing)" button.  Your "Paypal email" is your Sandbox Business account.  Do not forget to click "Save Changes" at the bottom of the page!  I did not take the time to play with the "Site Paypal Logo" option.  That's it for integration!
• Tester/Client Account
  If you do not already have a test client account log out of your site or open a different browser and browse to your web site's client login page (e.g., http://<yoursite>/please-log-in/) and setup an account using the same email you used above for the Personal Sandbox account.  Check the email inbox for the welcome email (hopefully it takes only a few minutes, so take a bathroom break or something), log in as the client and buy something.

You can view your Sandbox Paypal accounts by logging into https://www.sandbox.paypal.com with your Sandbox accounts.

Now you can see what the experience is like on your web site as a client.  You may want to put on your site the fact that your site is a test site.  I am not sure what happens when someone with a real Paypal account tries to purchase something, though ... you could disable registration while using the Sandbox (from admin dashboard go to Settings->General and deselect the checkbox "Anyone can register."

Of course, once you are ready to sell your pictures for real make sure people can register, your Symbiostock->Settings Paypal section is set to, "Live," and your real Paypal business email is in the, "Paypal Email," field.

Edit:  Updated based on helpful correction by Redneck, thanks!

Using version 2.2.7 and not sure if this classifies as a "bug" or a design question for the community (or door number three?).

It seems that only a user with an "Administrator" role can have a Symbiocard associated with their pictures.  What if someone wants to create a site for someone else and only give them an "Editor", "Author," or "Contributor" role?  Not sure

Is the intent of Symbiostock to be one artist/photographer per site with a network connecting/associating the different sites?  If so, it makes sense to have one Symbiocard per site.

For that matter, how are the different Wordpress roles "supposed" to map to Symbiostock permissions in the backend?  For those (like me) not familiar with Wordpress already the different Wordpress roles are defined at http://codex.wordpress.org/Roles_and_Capabilities.

From what I can gather here are the current back-end permissions for the different roles:

• Subscriber - Can only edit their profile in Symbiostock back-end.
• Contributor - Can do Subscriber stuff plus create new "Post," "Image," "Comment," and "Datasheet" content and edit their own content, but cannot publish them--"Submit for Review" is there instead of "Publish"--nor can they upload new image files of any sort.  They can reprocess their own Image content from the Symbiostock Images section of the backend and insert files in the Media Library into the Description section of an Image.
• Author - Can do Contributor stuff plus add new Media to the Media Library (not proper Symbiostock Image content) and publish/edit their own content.  This means they can put a new picture into the description for content of type "Image," but not upload something that will be delivered automatically and "processed."  Not sure how Paypal works with Image content created this way ... but proceeds probably go to the person that setup the Paypal account.
• Editor - Can do Author stuff plus publish/edit stuff submitted by others.  No access to Symbiostock upload features.
• Administrator - All access.  Can use Symbiostock Upload.

FWIW ...