"Just add 30% no big deal" That is a common comment left when someone, especially someone new to the industry talks about commission fees. I am always puzzled when someone just throws that out there.
1) This is not a booming global economy, companies with far more vital goods and services find it hard to pass along 1 to 3 percent increases.
2) Price matters, you cannot say a domain just goes up in asking price because of the venue where you are selling.
3) End users do not care about your selected venue and what they charge you. I spoke to someone who actually sent that back in the comment section on SEDO. He told the end user that they have to understand that Sedo charges 20% commission. He told me after he sent that, they ended the negotiation.
You need to think about what you feel your domain is worth, understand that if you use a secondary exchange like Sedo,Afternic or Go Daddy that there is a commission and maybe slightly bump your price. You cannot think that "Oh what the heck is another 30 % ?" 20 to 30 percent is a big deal in a lot of transactions.
You need to think about having your own sales site set up and using Escrow.com to handle your sales so you can get the price you want without having to worry about another 20 or 30 percent negating the deal.