You have a bit of a chicken and egg problem getting started.
The people will the really good domains that you'll want to market for a decent price (and therefore a decent cut to yourself) will be very wary - a poorly executed outbound marketing approach could expose them to legal risks that they are not currently incurring. For a <£50 domain, who cares. For a valuable one, it's a big deal.
On the other hand, the people with portfolios full of stuff that won't sell will be all over you like a rash because, well, why wouldn't they?
So you have to find some way of establishing a credible track record. Perhaps if you can take on a handful of domains and have them rank on DNJournal's weekly domain sales charts, that will start to spread your name around and build your profile.
Remember too that if you're working for the seller, you can't also be working for the buyer. Yes, I agree that it's important to manage expectations up front, but if you are repping on behalf of the seller it's your responsibility to protect their interests, not go back to them with a sob story about how you've found a buyer but their pockets are just too shallow so please accept this cut-price offer, etc. etc. In other words, choose your side and stick to it. IF you work on the seller's behalf, you have to do everything in your power to justify the seller's price expectations and find somebody willing to pay that price. That's what the seller is engaging you for, after all. If you think you've found a buyer but they're sticking to their guns nowhere near the asking price, they're not a realistic candidate - so you'll have to find another.
