As
@AstronomerXI noted, US Audio Mart is a good place to start. Free, and gets a good bit of traffic. I've bought and sold several times through them with good experiences. Audiogon used to be a great place, but their fees kept climbing and I don't know what kind of traffic they get now. Haven't visited in quite a while. Worth a look, perhaps. There's always the classifieds here at Head-Fi depending on what you're selling. It's also free. Ebay will get you the biggest audience, but they have the highest fees and also the biggest audience of scammers. Ebay is also now sending the IRS your sales revenue information so that income taxes can be collected if you sell in excess of $20k. Probably not an issue for most of us, but the IRS is also considering changing that threshold to $5k for 2024 (this directly from Ebay's FAQ page), so just an FYI. Ebay does have both buyer and seller protections in place, so it may be worth the price of their fees if having that is important. I've never sold anything through Amazon so can't comment on that.
The price to charge is hard to answer. Depends on what it is, how popular it is, how old it is, etc etc. Some things can get you 85 - 90% of the original selling price, and some things you may have to price at 50% to get it to sell. Best thing to do is scout the sites for comparable pieces and see what they're being listed for.
I always use PayPal for these transactions. Just what I do, not saying it's the best or only way. PayPal has protection mechanisms in place that can help you from getting burned, although even with that there are no absolute guarantees. My only advice here is that if anything during the process with a potential buyer causes a red flag to pop up, listen to your instincts and walk away. There will always be another buyer. Ship only to the registered address they have with PayPal (or whoever you use for payment) -- never ship to a different address ("I want to send it to my buddy's house so my wife doesn't see it" kind of crap). Major red flag.
Shipping only CONUS is your call. Much easier with fewer chances of problems and/or having to deal with customs/fees/duties, but you'll limit your audience a bit too.
Lastly, make sure the item is well packaged for shipping and be sure to insure it with a delivery signature required. Even if it's only an item you're selling for $100, spend the extra few dollars to be sure you have a good paper trail.