Is it b2c or b2b? imo/experience, b2b SaaS are the only ones that ever succeed. If not subscriptions, how do you generate revenue? Really it's either build an audience or cold calling. Note: I've never succeeded with any of my startups (for a bunch of reasons) so take with a grain of salt.
So a marketplace kind of app? (if so, you now have to build two audiences!) Do they self host after the sale? (Friction for b2b side). Is it enterprise (eg high touch sales - leads to long sales cycles)? Pay per post?
At the end of the day, it comes down to marketing and sales. Marketing:
- Craft the messaging around the pain point you're solving
- word of mouth
- advertising (there are different kinds, not just cpc/google adwords: guest posts, external content sites/blogs/channels in your market/demographic, etc)
- organic search (content marketing - this takes time - AND you have to promote the channels you are publishing too)
- cold calling
I'd be very nervous if somebody told me they will host my business data or otherwise provide a service to me forever for a single time payment. Maybe that's what makes your customers go away.
It's both B2C and B2B. And it's a one time payment.