I have no problem paying a small fee to use this service!
Why would you pay when it's going to be free? No fees is what makes this great as anywhere that charges a fee will just likely rip you off like ebay and paypal does.
it depend on the amount, if everything they will ask will be around the same fee that you pay for a transaction, say the usual 10k satoshi, i don't see any problem in paying that fee if it can help arbiters or escrowers or an automatized bot for securing both buyers and sellers