Why do I need to validate EU VAT numbers for B2B sales?

When you sell goods B2B to a customer in another EU country, you can generally zero-rate the supply as an intra-Community supply — but only if the customer has a valid EU VAT number (validated in VIES) at the time of supply, you report it on your EC Sales List, and you can evidence both the check and that the goods actually crossed the border. For cross-border B2B services the mechanism is different: the supply is outside your country's VAT and the customer accounts for VAT under the reverse charge. If the conditions aren't met or you can't evidence them, the tax authority can deny the zero-rating and assess the VAT against you.