Craigslist or your local Mac User's Group would be a few good alternatives.
I have found that if you do not have an unrealistic price in mind that eBay is still a good place to sell. Something that I do in my auctions is to set a decent "Buy It Now" price and as an incentive to have buyers use that option, I will give free shipping or throw in the extra RAM that I might have about. I of course take all the fees and shipping into consideration when setting my price, but it has worked out fairly for me - as well as the buyers.
Basically, you post a message in the for sale part of the forum (works the same as the method you used to ask your question here). And, then describe what you have for sale, include pictures if you want, and say how much, and how to contact you.
I have no connections with any of those sites. But, I've used almost all of those sites at times to sell different things.
I've never had a problem with any of them. But, like anywhere else, you have to watch out for the people who might take advantage of you. Basically, get money up-front before you ship.
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Places to sell g4?
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