The usual:

USA oriented:
http://www.vcfed.org/forum/forum.php

UK/EU oriented but there are also US sellers and most non-US sellers also ship to US:
http://www.amibay.com

Depending on where you are, Craigslist frequently has listings that may be interesting - although you're less likely to get good information, prices will usually be significantly cheaper than on eBay ("Big lot of scrap computer stuff for USD 20" is more likely than "Abit VH6T tested USD 200").

Go to a search engine, type some keywords, hit the enter key and browse. In Germany you can find several big webshops, but I’m sure you can find such webshops in the U.S. as well. You might not be able to find the specific motherboard that you’re looking for, but you’ll find lots of socket 370, even some NOS.

You can indeed also buy lots (of private sellers), but you have to be careful because some folks know very well what they are doing (e.g. dumping the motherboards with leaking caps online and keeping the good stuff for themselves).

You see the latter everywhere. 9 times out of 10 "untested" really means "tested and dead as a doornail". That said, so long as the seller is honest about it, it's fine. I'll happily buy 'dead' boards with bad caps as I know what's wrong with them, I know I have the skills to fix it and I know over half will work flawlessly after doing so.

And yeah, guilty as charged for dumping stuff that's not interesting to me in larger lots if I'm not able to sell it individually on say Amibay. If the alternative is binning it, I'd say this is preferable - so long as information regarding status (working/not working/not tested) is accurate.

Hello,

I have an issue with my Amibay account and I couldn't get in contact on admin@amibay.com .
Can someone help me, please?

Thank you!

^ so who is Tweety_XTC ? your old account?

Hello keropi!
I am really happy to get in touch again. We talked last year in april about your card. Unfortunately I didn't have enough time, but now I plan to continue my build.

Regarding my issue, first I have created Tweety_XTC on the go but I didn't pay attention and something went wrong (I rushed through the activation). After that, I have created the second account thinking I could delete the first one afterwards (stupid mistake).
As I recall, both accounts were created in the same day and I didn't get to use any of them. I absolutely didn't have any intention to perform harm and I would like to apologize for the inconvenience created.

Thank you very much for your assistance, I appreciate it.

Blu_Wave

I see
check Blu_Wave account again for email confirmation

Thank you very much! I appreciate your help.
I had to manually request the confirmation email and it worked perfectly.
Keep in touch.

Best regards,
Blu_Wave