In real world, startups are generally acquired by the private firms or some independent investors. If you are having a startup and that is sustainable and you want to sell it, then you can connect with private firms or any online marketplaces. They will definitely help you out to find a buyer.
The selling decision is totally yours. I just wanted to convey you that , longevity is the only key to startup success, so take your time, do your research and then go for a decision.