First do you have to be a registered business to become a seller?
# No you don't have to be, although if you are trading you should be at the very least registered as a business with your local tax regulator. In the UK that would be HMRC. We have advertisers who are sole traders, we also deal with small to medium sized businesses.

Second can you be denied after you pay the fee?
# You aren't invoiced until/unless you are approved as an advertiser. If you are removed as an advertiser after you have paid the fee, there will be a reason for this and it probably won't be on the best of terms. (In otherwise your access will be terminated).

Third how long does it take from the time you apply until you can sell?
# This is answered on the relevant Advertising page.

Should you have any further queries I would be more than happy to try to assist.

Many thanks.