Im my experience Sedo doesn't prevent other offers to be sent to you while you negotiate that same domain listing, once you accept/auction ends then of course you're bound, I don't believe Sedo would present any offers to you coming thru them after that, while you wait for payment. What is worth noting is that after you accept an offer to sell (or your auction ends) Sedo won't present any offers to you whatsoever, sometimes buyers are given 30 days to pay if Sedo wants them as clients and/or sellers don't ask Sedo cancel the transaction after so many days have passed. If I were a seller I would park that name elsewhere (or at least direct that domain AWAY from any Sedo servers altogether) once you "sell" it, that way if he doesn't pay (buyer) you continue to get offers on the parked page FOR SALE link (make sure there is one) just in case people are too lazy to use or don't know about the WHOIS (for some reason in the last year or two people buying from me on Sedo and not paying later has become a little more common)
After so many days of negotiation that doesn't succeed Sedo emails sellers the offer is not binding anymore. At that point you can cancel the negotiations formally w/that particular buyer, but it's a bad idea, twice or so I've had that same person come back to me later with a higher offer and the names got sold, so just leave them there, they're not "open" as far as Sedo is concerned