You will need to register and identify yourself.
This usually means adding a Paypal account (if you don't already have one) and givign some financial /credit information, usually bank account (the same info that is on the bottom of every cheque you have ever written so don't panic) and credit card info (the same stuff that is on every cc slip you have ever signed so again don't panic.)
Paypal is very secure and will not release your information. Most buyers prefer to use it since it is free and fast, and hides their financials from sellers. There are other methods of payment allowed, but most buyers don't like or trust them.
And those 50 Free Listings? You can get another 50 by putting the next batch on dotCOM instead of here on dotCA. The only difficulty is that on dotCOM we Canadians have to use Flat Rate Shipping, while on dotCA we can use Calculated Shipping, but for stamp sellers that is usually not a problem.
If you are offering albums or other bulky items, put them on dotCA where you can be more precise about shipping costs.