Myspace and Paypal
It appears that Myspace have deliberately broken 'Buy Now' buttons so that they no longer work. Searching on Google sugests that this has been happening for just over a month, though most people are still unaware of this.
I only found out this morning after a local musician contacted me to say neither of the ones on her page were working. The one has been there over a year and used to work, the second I added last night for her.
For musicians who have their own websites as well the solution is fairly simple, just edit the button link to redirect to their own merchandise page, as I have done in this case. Unfortunately I am not aware of a simple solution at the moment for the significant number of musicians who do not have a suitable page elsewhere.