Just another thought on your comment about computer consultancy and doing bids and such. Agents do put in up front work in coming to do the listing appointment initially to list the home. Typically, they've spent time putting together a pre-listing packet/listing packet, do CMA's (comparative market analysis) on your home to see whta's selling in the area, researched comps and the like. Then, They come to the client's home (commuting) and spend time presenting the materials to you. All of that work is usually done gratis in hopes of finding a mutually beneficial relationship. This is akin to the proposal you put together with your clients.
However, once you engage them, then they are working for you. They spend money on listing the home on the MLS, flyers, as is the case with us extra features to get your home propagated on dozens and dozens of sites throughout the Internet, sign for the yard, lockbox, showing service fees, specialized websites just for your home, 800 phone numbers to capture names/numbers of people calling about your property, possible open home paper work, refreshments, etc. and on and on. When an agent takes on a listing, it's only smart business to take on homes they think will sell as they put in all their own money up front with the probability of getting reimbursed for the outlay they've already put out.
So, at this point, it's more of an ethical question.
Hope that's helpful,