Every rail commuter knows to stay away from the edge of platforms: and the same applies to tax, too.
The metaphorical (and digital) "platform" in the case of All Answers Limited V The Commissioners For Her Majesty’s Revenue & Customs  UKUT 0236 put students wanting someone to write their essays and dissertations in touch with people prepared to do so.
In this case, the VAT issue was whether, as All-Answers argued, it only had to account for VAT on the amount left after paying the authors where it was acting as their agent. Conversely, HMRC argued that the entire fee paid by customers was received by the platform as principal, because it contracted with the author – hence VAT was due on the full payment.
The problem was that the contracts between: (a) the customers and the platform; and (b) the authors and the platform, were not wholly consistent with the agency argument. In part, this was because it was key to the platform's business models that authors were anonymous, in order to "cut out the middleman" (and so that the authors' employers did not know that they were "moonlighting"). The principal reasons were that the platform had obligations to the customer itself, and not just as agent for writers. In particular, the copyright in the essays was transferred to the platform itself.
Platforms can successfully establish that they are agents for VAT purposes where the proper agency relationship is established. Often it is the tension between the platform wanting to quality control for its customers, but needing to demonstrate that it is not the supplier.
Therefore, it is strongly advisable that such platforms invest in high quality contracts and legal advice, even at the outset when they are tempted to minimise start-up costs. The FAQs for their websites, their actions and other materials must also be internally consistent. As other well-known platforms are finding, HMRC will be assiduous in challenging structures that appear to confer power without tax responsibility.