They should not need to get this info, but generally the person requesting it is simply acting on a policy where they have no discretion. What the policy makers are concerned about is distinguishing between employees, contractors and other entities. You may also receive a 1099 from them at the end of the year which is a pain because you will have already declared the sales as sales and the 1099 duplicates that record.
Here what the IRS says:
Form W-9, Request for Taxpayer Identification Number (TIN) and Certification
Use this form to provide your correct TIN to the person who is required to file an information return with the IRS to report, for example, income paid to you, real estate transactions, mortgage interest you paid, acquisition or abandonment of secured property, cancellation of debt, or contributions you made to an IRA.
Clearly, this does not apply because the payment to you is not INCOME that needs to be reported to the IRS, it is a purchase but I have had very little luck convincing the people asking that this is the case.