Backdoor selling is the practice of asking innocuous questions of your non-procurement people, in order to influence them to buy your services without competitive bidding. As you can see, it is generally not a fair process and is even illegal in some places.
For example…
Let’s say your company’s procurement team was negotiating with prospects for a solution. You being one of the key consumers of this solution are not involved in negotiations but are aware of the state of play. If one of these providers were to directly ask you questions like “What are you looking for in a provider?” or “What is your budget exactly”, that provider is trying backdoor selling tactics on you.