HI Jim,
I do not recommend Six moons, they are way oversaturated with ads as it is, and I think it would be hard to get noticed.
PFO is expensive, tough to get good placement on, and does not give good ROI. I have a banner running there right now. It gives me 6 click throughs a day, and maybe one or two customers a month. I will not be renewing my contract with them.
Audiogon is by far may favorite. I can tell immediately by the jump in web traffic when I have a banner or classified running on Audiogon. for about $200 per month, you can get into their program. That gives you $200 worth of ads per month, you pick where you want them, and what type you want (standard classified or banner).
An interesting aside. If you run a classified ad on Audiogon, you are placed in the Stereophile.com marketplace. Its a cheap way to advertise to the Stereophile crowd. However, if you run an Audiogon banner ad, they are stripped out of the feed to Stereophile. So the lesson is run banners alongside regular classifieds for maximum exposure.
Audioasylum would be good for DIY, and its cheap.
Google adwords can get you wide exposure as well, but I am still analyzing the effectiveness for our particular niche market.
I hope you find this helpful, PM me if you need any more information.
Mike