You can read about 1099-K reporting requirements and the need for your social security number directly from Amazon Payments here:

https://payments.amazon.com/sdui/sdu...al-Regulations

There is a list of FAQs that should answer most of your questions.

Here is the first portion of the link:

"IRS Reporting Regulations on Third-Party Payment Transactions

Beginning with the 2011 tax year, new Internal Revenue Service (IRS) regulations require that U.S. third-party settlement organizations and payment processors, including Amazon Payments, file Form 1099-K to report unadjusted annual gross sales or payment volume information for customers that meet both of the following thresholds in a calendar year:

More than $20,000 in gross payment volume, and
More than 200 transactions.

IMPORTANT NOTE: Even if your account does not meet the above criteria, Amazon Payments still requires you to provide tax identification information to Amazon Payments."

Regarding your TIN/social security number. IMO, if you don't have a TIN/SSN on file with AP, it appears they would have to withhold 28% (backup withholding) from your transactions. You don't want that and neither does AP. The easiest thing for them to do is to deny payments for anyone unwilling to furnish a TIN/SSN.