Give them your SSN but change the last four digits around, for example, 1036 could become 3601. PayPal can't verify a social anyways, it's just for their records, and mostly for security. There's actually a lot that goes into a social security number so you can't change around all of the numbers. The first group explains where you were born by region, the second by state, the last four are assigned "randomly."
Or, get a EIN.