e-Bay is best, but make sure to drive some sales there by letting folks know on appropriate forums and so forth that the item is there.

There's a couple vintage synth forums online and it would behoove you to become a member there first.

Vintage mixers; for recording use, unless it's one of the big desirable names like an SSL, Neve, API, etc. - you probably will have some difficulty there. There is a market for old Mackie and Allen & Heath mixers for road use..

Again, do your homework on active forums as to pricing and so forth, then be reasonable about your initial pricing on e-Bay based on your research at forums, what past items have sold for, etc. Then on those same forums, let folks know that you've got something they want for sale on eBay and put the link in the forum post.

Don't put a minimum bid amount in. Put the lowest amount of money you are willing to accept for your gear into the initial bid price and let things go from there. You may be pleasantly surprised.

I did this for a pick that was thrown into the crowd at the 2003 G3 concert - a Joe Satriani pick. I put the pick up for sale, along with my ticket stub, after joining the Joe Satriani website forum. I put up the pick and stub on e-bay, then let the folks know at the Satch forum that it was there.

I cleared $41 and some change after several fans tried outbidding each other in the last 1/2 hour of the auction. I started the bidding at $5.

Make sure you check on shipping price for your items with USPS, UPS or FedEx to the farthest point away from you in the US (I recommend only selling inside country) and list that as the shipping price. I got burned on an Alesis effects unit I sold (first thing I ever sold on eBay) when the price I accepted really didn't clear me much more money than what I paid for it as well as the shipping cost (I paid Mailboxes etc. to pack it - big mistake).

-Scott