Section 6a.1. (13) UStAE

(11) A person/entity acquiring the object for his or her business must be an entrepreneur at the time of delivery. It is not necessary for this entrepreneur to be based abroad. He or she may be an entrepreneur based in Germany. It is also irrelevant whether the customer is a small business owner (possibly under the law of another Member State) who engages exclusively in tax-exempt transactions for which no input tax is deducted or a former or forester who taxes his or her transactions based on a flat-rate system.
(12) If the buyer provides a valid VAT ID issued by another Member State to the supplying business, it can be assumed that they have business status. Mere proof of the submission of a request for a VAT ID at the time of the transaction is not sufficient. The VAT ID must be valid at the time of the transaction.
(13) It can usually be assumed that a buyer is acquiring an object for his/her company if he or she provides a VAT ID issued by another Member State that is valid at the time of the transaction and if the nature and quantity of the objects acquired do not justify doubts as to their use for the buyer's business.

Section 3a.2. UStAE

If a service recipient based in the European Community area does not provide a VAT ID to his or her service provider, the service provider may assume that the service recipient is a non-business or an entrepreneur who is procuring the service for a non-business activity, unless he or she has information to the contrary (cf. Article 18 (2) of the Value Added Tax Ordinance (EU) No. 282/2011, loc. cit.). In this case, the place of performance is determined in accordance with Section 3a (1) UStG, unless Sections 3a (3) to (8), 3b, 3e or 3f UStG apply.