You can only sell your domain name if someone wants to buy it!
If someone searches to see if the domain name is available, and finds that it is not, they can always contact you via the domain name registrar to see if you're willing to sell the name. The fact that, presumably, nobody has contacted you up to now, probably means that there's nobody out there who wants to buy it!
Most companies who sell domain names register several thousand names at a few pence each. (They know what people might be interested in, so they register, for example, poodles.com, setters.com, collies.com, spaniels.com, etc, etc. Most of these names they never manage to sell but, sooner or later, a dog lover comes along who regards poodles.com as a name they can't be without. The company then sell this name at a price which covers the outlay on a thousand domain names). So the chances of a single domain name being one which a buyer might be interested in is very remote. (As, indeed, is the chance of finding any firm prepared to act as an agent in your attempt to sell a single domain name).
Chris