That is actually the point of using an escrow service such as escrow.com for domain transactions. Escrow.com in essence makes sure each party has the proper outcome.

"the owner can ask for a different price based on who asked"

Nothing prevents the owner, when dealing with a middleman, from taking advantage of the buyer even with Sedo in between.

At a certain point in the transfer process with SEDO the new owner of the domain is revealed. Nothing to prevent the seller from deciding to not sell the domain. Doesn't even matter what the legal status is. Fact is nobody is going to sue over some small amount of money. [1]

Let's say you own "slig.com" domain. You decide to sell it by way of SEDO. You list it for $2500 which you think is a good price. Then you see (later on) that Facebook is the buyer. You decide "hmm maybe I should not sell it" and you find some way to get out of the deal knowing that facebook probably isn't going to file a lawsuit to get the domain but will most likely decide to give you more money and just be done with it.