It looks like they've changed it since I first applied. I filled in an online form, then didn't hear anything for two weeks. Then asked on their online chat, where they said somebody would be in touch. Still didn't hear anything two days later. Then emailed them about it, and they sent me the relevant info.

It looks like you can now just go right ahead and set yourself up.

You only get 1% of a standard sale compared to 10% on Amazon, so I'll be using a mix, e.g. it suits me better for people to buy strings via an Amazon affiliate link, but if people are looking for a discontinued guitar or amplifier, Reverb or eBay need to be options.