I'll do lots on ebay if I have a bunch of cheap DX r beat-up Champ discs. It's easier to sell cheap discs by the lot, 'cause it cuts down on the S&H per disc.
I haven't had any luck selling premium discs by lot, 'cause the bargain hunters don't want to pay much for a lot. They assume it's all junk, because it's in a lot.
I hear the highest prices are on Facebook right now, but I adamantly refuse to open a Facebook account, so I'm just going on what I hear.
The next outlet is ebay, but you still have to pay a 9% commission.
Next best is probably the DGCR Marketplace, but whether or not you get a good price depends on how in-demand the disc is. Throwers are not getting much right now on the MP.
Last, take to the local PIAS, if you have one.
As far as shipping, USPS First Class in a bubble mailer is the best, easiest way that I've found to ship single discs. It usually runs me about $2.40 to ship a disc, plus the cost of the bubble mailer.
The above advice is not meant to be comprehensive, it's just a reflection on my experience.