1. He/she could be scrambling to raise funds, oftimes by having their own domain fire sale.
2. He could be looking to flip the domain, by first tying you up in a contract and then looking for a secondary buyer.
3. I've never been a big fan of brokerage services, as the few times I've used them on the buy side I've had problems.
4. Could just be someone stringing you along. That's why I use a very short leash when making a sale: Payment in 3-5 days or the deal is off.
Just tell the SOB the deal is off for failure to make timely payment. IF you have a truly motivated and good faith buyer they will find a way to wire you the funds at that point. Don't accept a promise of payment at that point. "Money talks, BS walks."
I once terminated a deal, canceled the escrow arrangement and told the buyer in an email I could only conclude they were a fraud after the several failures to deliver on their promises. The next day the money $$,$$$. arrived in my personal account, no escrow (1 of 2 original options) followed by a profusely apologetic email from a senior partner of a company advising that a certain incompetent subordinate was removed from the transaction.
Sometimes telling a buyer to go to heck is what is needed.
The problem with some escrow services (ie Sedo) is they don't tell you who the buyer is and you are legally bound to a buyer/seller contract that has no time limits to the transaction.
My advice is to ask the brokering service to cancel the transaction (as I did).
I had the same problem with a $nn,nnn domain last year (and, with the same domain and brokerage, the previous year, where the buyer eventually backed out with no penalty).
This time, I didn't have as much patience and cancelled the contract after three weeks (they had less time than that to pay, in the contract).
If it's a good domain, there's always another buyer later. I sold the domain privately later for more. And removed all the rest of my domains from the brokerage.
In the age of electronic transactions any payment beyond 10 days is a sign of trouble. Any brokerage agreement ought to provide - by default - that if payment is not delivered within 10 days the offer to sell is terminated.
Personally, absent compelling proof that I'm dealing with corporate chain of command issue, I don't allow 10 days in those rare cases where I sell a domain.
A serious buyer will be able to deliver payment within 3 days of agreeing to a deal. I've had serious buyers FedEx bank checks overnight - from the day we struck a deal based upon a telephone call.
It's a seller's market, IMHO, for quality domains. STOP letting the buyers or brokers dictate the terms of sale.
Frankly, for quality domains brokers are entirely unnecessary. Buyers will find you and, at most, you'll need an escrow service - that doesn't rake off a percentage of the sale price - to close the deal.
Sellers - Stand up for your rights and start stating your protest! Demand that brokerage form contracts become more seller friendly. Make your sentiments known.
(Smiles benevolently at audience. Closes Domain Bible. Steps down from pulpit.)