I used to remind my realtors that a verbal offer isn’t worth the paper it is NOT written on. You might suggest to the prospect that even if the seller would be agreeable and say yes, there is nothing binding. They could change their mind or mention your “offer” to a friend or relative who will then offer more. Should, however the seller sign your written offer, it is binding.
A written offer (contract) is so much stronger. A written offer with earnest money is definitely stronger. A written offer with a confirmation from a lender and earnest money is still stronger.
We are in a litigious world today and it is most important to remain safe. Should a prospect declare that they did not give you the right to negotiate on their behalf or present the verbal offer, it is your license on the line. The burden of proof would be with you, or guess what? You just bought a house.
Often, a transaction that begins with an emailed legal offer winds up in negotiation. Once again do not verbalize. Be certain any changes, no matter how small (a closing date for instance) or the inclusion of an extra piece of furniture needs to be agreed upon by all parties IN WRITING.
Certainly you would not accept a verbal commission check, would you?