Buidlers can protect themselves with staged payments or gettign the customer to buy large orders direct if they wish.
I think you meant to say "and" getting. Clients buying themselves lots of stuff to use themselves is no protection for the builder.
Attempting to lure builders with a heavily biased incentive in favour of the client, is not going to garner any interest from busy builders. In particular now that the building trade is buoyant again.
We are turning work down unless clients are prepared to wait 6 months or so. Once a builder is in a position where he can cherry pick work, then the awkward jobs stay at the bottom of the pile.