What can you do about a nonselling seller?

Forget about it. It's irritating but life is too short to worry about such things.

I think the best thing to do with a non-selling seller is not to buy from him,if nothing else it'll confuse him.
:-)
Seriously though even though I've never bought anything here on Audiogon since I'm in the UK-you must be aware it is the nature of the beast- selling and buying privately.
It's a sad fact of life that you'll get time wasters,people who blatantly lie and potentially a fair bit of grief when you try to either buy or sell most things-for every decent person you come across(especially when selling)you'll get five idiots.
Herman makes a good point and in the grand scheme of things a lot worse things can happen than what has happened to you.
Hope you get another power cord-Lordgorian.

I agree with Herman: forget about it; life's too short.

For the sake of analysis, my guess is that this seller received an offer before yours. He agreed to sell it, and the buyer paid via paypal, overnight delivery of cashiers check or money order, etc. Until he could confirm payment, he decided to string you along with a delaying tactic. In other words, if the first buyer didn't pay up, he would still have you as an interested buyer. If he had told you the cable was already sold, you might buy one from somebody else (e.g., order it from a dealer) before he could confirm payment from the first buyer. While this is not a nice thing to do, he technically did not lie to you. It appears the first buyer came through with the payment, so the seller was willing to come clean and say the cable was already sold. Find another seller!

If you are a serious buyer, I'd also recommmend you show that you are by giving your complete name, phone #, and delivery information and inquire exactly on how you will be paying for the item. That will get a sellers attention and show that you are serious, instead of, "Yeah, I want to buy that."
I've heard that too many times, on items offered, and most of the time, they don't close the deal when that is all that they initially say.
...and sometime, even when you show that you ARE serious, some sellers will change their mind AFTER making a committment to sell an item to you. I recommend you accurately and honestly post feedback against them when they don't live up to any firm commmitment made.
Good luck and Happy Listening!