Craigslist, eBay, the seller section in this forum. And other forums. Lots of places and ways to sell used gear. As long as you have some means to quantify the chain of ownership.
As far as it's value. Outside of some lost hollywood treasures that have your mic starring in a tv series or late night talk show. You're probably looking at 1/2 MSRP. If it's a popular mic, you can get higher than that. If it's brand new-ish or if it's rare and popular perhaps better still. But generally count on 1/2 MSRP or whatever similar items go for on eBay. And you can't really go by the manufacturers MSRP, but by what you can get a NEW one for ONLINE. If you can't get a new one, that helps make it special, but any references to it's MSRP will not be adjusted for inflation, so you don't always come out ahead.
If you deal locally (craigslist) you can factor in S&H in part and get a little more for your gear. Or just sell it faster since it'd cost more even with the little extra more to buy it elsewhere. But it really depends on how badly you want to sell them, or how fast. I tend to keep most of my unused gear fairly actively listed on craigslist. If it sells it sells, if not, well it's not going anywhere. Although if things get any tighter I'll probably be willing to drop the price to have the cash versus unused gear. And some of the less popular stuff I generally have to bundle with other things before someone shows any interest.
