I think internet marketing is a good idea, you don't need any capital to sell the products or services you only need to do is to introduce and promote the product or services you offer. It is a good source of Bitcoin, you don't need to invest you can only need is to get the trust of the buyer and consumer to have a good sales.
I think you still need some fund here. You cannot advertise your services fully if you do not subscribe for some service ads booster. Aside from that if you want to set-up your own site you need to pay for hosting, domain, ssl cert and the templates. Though you can advertise on social media platform but if you do not avail for boosts then you will only reach limitied audience.