I have been in this situation but selling to the States. Nothing happens if you sell to end users who pay with credit card but as soon as you have a large sale to a company or institution they will ask for all kinds of documents that force you to register properly and pay taxes.
Your client will ask you for a document (can't remember the details) and to get it you have to register with the IRS. If you pay taxes or not, I'm not sure, the sale was too small and I just dropped the client.
