With just once chance to submit a counter offer the seller I believe would make a more considered and realistic counter. If they could submit multiple counter offers, then why not try you luck with a whopper, if you know you can follow up a couple of months later with a 'realistic' counter.
Um... just so you know this is exactly what I do right now, but just on a 7 day timescale
Let's say someone bids $100 for a domain (let's face it most first offers are around here unless the domain has previously had offers) for my first counter offer I work out what the rough retail price for the domain would be, let's say it's $1000 and I up it by 50% (that's my whopper markup which gives me a bit of room to negotiate as well), which would mean I counter offer with $1500. I normally try and do this within 60 minutes of receiving and offer.
Then I wait.
If you have a serious buyer that sees value in domains they often come back with something in the $300 to $500 range at which point you have yourself a negotiation and with any luck you will find yourself selling for somewhere in the range of $750 and $1250.
If after 2 or 3 days you haven't received a counter offer what I often do is send through ANOTHER counter offer (which netfleet allows you to do - I don't know if you meant this to be the case but it is) and I assume this sends them another email, which I see as a bit of a prompt for them to reconsider. This time I probably offer $1200, which shows them I'm willing to negotiate but I'm not willing to sell at a 80% discount or anything (ie I'm not selling for $200).
I'd say reducing the counter offer price brings the buyer back to the table about 30% of the time even though I've only reduced the price by 20% and they will usually bid something a bit higher, sometimes they go to $150 to $250 (that said sometimes it really is a case of they just missed the last email or forgot and they end up sending through a decent counter offer). At which point I counter again this time at the 100% of retail price, which would be $1000.
Finally if we still aren't getting towards the same ball park I often end the negotiation with a counter offer at 25% below my expect retail price, which would be $750. Then I just call it a day and figure if they aren't willing to buy good value when they see it I'll just have to wait until someone who sees the value comes along.
The point is I already submit multiple counter offers and everything seems quite happy and amicable with my process when I talk with people via the Q & A or when a sale is made.
My only disappointment is that negotiations can stagnant ie the buyer bids $300 when I think retail is $1000, and my final offer way $800, so the deal isn't done, but I get frustrated when in 6 months time my need to raise capital might be higher and the buyers perceived value of domain names has probably increased and I see all these previous offers which in the past weren't in the ballpark of what I was willing to sell for now being not too far off. So I just which I could rekindle the lead by sending through a quick, easy, painless new counter offer of say $550 a couple of months later, which might get snapped up by the buyer for all we know and both parties could be very happy with the outcome.
Then of course there is moronic situations where I get an offer of like $750 for a domain that I think should retail for $1200 and I leave my final counter offer as like $1100 and get all stubborn about it only to realise a couple of weeks later I was a moron and should have taken the money.
Or worse you counter with $1100 for a domain you value at $600 hoping they will accept, but are happy in the knowledge that you have an offer of $750 which you fully plan to accept (given you have 7 days to accept) if they don't accept your $1100 counter, but then you forget to accept it 7 days later and the offer expires and they don't counter offer or accept your counter offer!
Point is, does it do much harm to allow sellers to also initiate new offers and to rekindle leads that are of buyers who are obviously interested in the domain and whom I'm sure would be interested to know if the sellers circumstances have changed?
At the end of the day the buyers have the exact same ability, ie they can make an offer which lasts 7 days which the seller sends a counter offer to which they don't accept so they just let the offers expire, before submitting another offer a week or two later. So does it hurt to give the seller the same ability?
I'm not saying you would promote this feature, and I still think the VAST majority of both buyers and sellers want to close the deal ASAP (that includes me as well).
And if you were worried about people abusing the system maybe you could limit the number of new offers they could send to previous bidders whose offers have expired to one every 3 months or something.
Anyway... please give it some thought.