They can ask, but there's no need for it. They need the W-9 if they are going to send you a 1099-MISC for the money they pay you. But they should not be sending you a 1099-MISC. First of all, a person does not send a 1099-MISC. Only a business sends a 1099-MISC. Secondly, they are not supposed to send a 1099-MISC for a purchase of a product. It's only for payment for services.
You can try showing them the 1099-MISC instructions that tagteam referenced, but they are your customer, so you probably don't want to get into a big argument with them. There's really no harm in them sending you a 1099-MISC as long as you report the income, which I assume you would do with or without a 1099-MISC. If you do fill out a W-9 for them, give them your EIN, not your Social Security number.
