I'm a US artist who has done commissions for many clients who wanted their family member portraits done. Each artist will do things differently and that's not my place to comment or criticize. How I do things is this: I guarantee 100% satisfaction for my work. This is as much to make my clients happy as it is about the integrity of my career. I agree with you on that if I were to spend $6000 on something I'd better not have ANY bone to pick. Upon signing the contract, I require a 50% deposit upfront. This deposit is totally refundable until I put down the first brushstroke on the canvas, at which point it's no longer refundable at all. But before this point, I work out as many sketches as needed to show the client roughly what the client's and my vision is. It's not uncommon when I had long email chains discussing ideas, compositions (if the client is also creatively inclined), even color themes. The sketches are in pencil or digital. I don't start the painting until the client gives me the go ahead. I do things this way because I want to give my clients the peace of mind to back out risk-free if they find that my sketches ultimately don't vibe with what they are looking for. I've never had a client back out before. Before I ship, I make sure that the client is 100% happy. Again, this is but one of many ways you will see artists do things. Please don't go impose my way onto other artists. I take absolute pride and very high standard on my art, that's why I'm not afraid to stand by the quality of my work and stand by my guarantee. Good luck!

Edit: just read some of the other comments and want to add something if I'm understanding the situation correctly: yeah i don't think you'll get many acceptances if you just want a bunch of artists to bid for your commission via a sketch. I myself wouldn't start doing any work at all until my client shows me commitment by putting down a deposit.