What is the relationship between a vendor and purchaser?

This trust relationship between vendor and purchaser dates back to the 19th century. In Lysaght v. Edwards (1876), 2 Ch.D. 499 (Ch.D.) Jessel, M.R. developed the principle this way: [The] moment you have a valid contract for sale the vendor becomes in equity a trustee for the purchaser of the estate sold, and the beneficial ownership passes to the purchaser, the vendor having a right to the purchase-money, a charge or lien on the estate for the security of the purchase-money, and a right to retain possession of the estate until the purchase-money is paid, in the absence of express contract as to the time of delivering possession. In other words, the position of the vendor is something between what has been called a naked or bare trustee, or a mere trustee (that is, a person without beneficial interest), and a mortgagee who is not, in equity (any more than a vendor), the owner of the estate, but is, in certain events, entitled to what the unpaid vendor is, viz., possession of the estate and a charge upon the estate for his purchase-money... “Valid contract” means in every case a contract sufficient in form and in substance, so that there is no ground whatever for setting it aside as between the vendor and purchaser - a contract binding upon both parties...