1. Make it so good people want to tell others about it. It can't be an "hmm, yeah I guess that might be useful later. I'll bookmark it". It has to be a "Woahhhhh I've been looking for this exact thing for years. So have my friends I bet. I need to use this now, and tell my friends what I found". (A hard ask, but it's nice to aim for that 'wow' factor).
2. Make the base product free, and politely refuse donations. Ask people to simply tell their friends about it. I think people kinda feel guilty about having a cool product for free, so they become evangelists for it, and help you sell it.
Having said that, it's good to get something to start off. Look at websites/domains that are related, and have been neglected... Send emails, make an offer, buy domain, get traffic. It's not a sure fire thing - some people still believe domain names to be worth millions whatever they are. But you can pick up some real gems. More likely to be sustainable traffic unlike tc/reddit/etc mentions.