You need to know the make and model. This info is usually right on the front, but it will also be on the back as well.
Search on eBay for your make and model, and choose "sold items". Don't go by asking prices which are often very high.
For antique or "vintage" stereo components that have not been restored, these typically must be sold as is or, "for parts or repair". These items sell at a lower price than guaranteed used and vintage audio.
Check Hifishark.com. As with eBay, search for your item ,and select sold listings. You can then sort by several factors including region or country, price range, and others.
For higher end audio, https://www.audiogon.com/ offers a bluebook lookup based on past sales. This is best for audiophile class equipment.
Forums like AudioKarma.com can also be a good resource. Searching is the best way to start, rather than just immediately posting the question, what's this worth". This will usually turn people off.
Sell to an individual, rather than a reseller. People in the business of reselling, must pay a significantly lower price in order to make a profit. If your goal is to get the most money for your item, sell to an individual.
Know the typical value, and ask a reasonable price. Starting too high will usually scare off buyers. Using a high eBay asking price as a guide is not the best idea. Instead, choose an asking price based on the average sold price.
Take some good photos. Use indirect light. Never direct sunlight or flash. This always brings out the worst aspects of appearance. Placing an item outside in the shade is a good option. Avoid bright or busy backgrounds. use a slight angle above, and to the side, rather than square on. Use Google to search some promotional images, and use this as a guide.
Describe the condition honestly, and what is included. It often adds value to have original boxes, literature, and especially important items like remote controls.
Facebook Marketplace is probably the most popular and successful choice for someone looking to resell a piece of audio equipment. Craigslist is a not as popular as it once was. Still a good option. No reason not to post on both.