There is a requirement that a registered VAT trader must provide a tax invoice to another VAT registered trader. There is no requirement to do so for someone who is not registered. Many people who buy things on eBay are simply "members of the public" who have absolutely no need of a tax invoice and it may be that despite what they say the sellers are more set up to deal with members of the public.
The relevant law is the VAT Regulations 1995 Regulation 13
Obligation to provide a VAT invoice
13. —
[F1(1) Save as otherwise provided in these Regulations, where a registered person (P)—
(a)makes a taxable supply in the United Kingdom to a taxable person, or
(b)makes a supply of goods to a person in a member State for the purpose of any business activity carried out by that person and P is identified for the purposes of VAT in Northern Ireland; or
(c)receives a payment on account in respect of a supply of goods that P has made or intends to make from a person in a member State and P is identified for the purposes of VAT in Northern Ireland,
P must, unless paragraph (1ZA) applies, provide such persons as are mentioned above with a VAT invoice.]
You can write to the suppliers who fail to this should you so wish drawing their attention to the law and if that fails contact HMRC for help! I'm pretty sure eBay will not understand or help.
@stv1962
