I've been seeing developers make money selling APIs on marketplaces like RapidAPI, and I wanted to try it myself. Here's exactly how I went from zero to two live API products in one day.
The APIs
TextPulse — A text analytics API with 7 endpoints:
- Sentiment analysis with per-sentence breakdown
- Keyword extraction with relevance scoring
- Readability scoring (Flesch Reading Ease + Flesch-Kincaid)
- Named entity extraction (emails, URLs, phones, money, dates)
- Language detection
- Extractive text summarization
- Full analysis (all of the above in one call)
ShieldCheck — A password & security API with 11 endpoints:
- Password strength analysis (0-100 scoring with pattern detection)
- Crack time estimation across 5 attack scenarios
- Secure password generation
- Password hashing (SHA-256/512)
- Email validation + disposable domain detection
- Domain breach checking
- TOTP 2FA generation and verification
- Secure token generation
- Shannon entropy calculation
- Full security audit
The Stack
Both APIs use the same simple stack:
- Python + FastAPI for the API
- Pydantic for request/response validation
- Railway (TextPulse) and Render (ShieldCheck) for hosting
- RapidAPI as the marketplace
- GitHub for version control
No machine learning libraries. No external API dependencies. No databases (yet). Everything runs as pure Python logic, which keeps deployment simple and responses fast.
The Build Process
The actual coding was the fastest part. Each API is a single server.py file with:
- Input models (Pydantic)
- Core analysis functions
- API key validation
- FastAPI endpoints
The time-consuming part was everything else:
- Configuring RapidAPI (adding each endpoint, setting media types, writing example bodies)
- Setting up pricing tiers
- Writing documentation
- Testing every endpoint through the marketplace
Pricing Strategy
I researched competitors on RapidAPI before setting prices. Key insight: most text analytics APIs only do ONE thing (just sentiment, or just keywords). TextPulse bundles 7 features, so I can price below the sum of individual tools while still being competitive.
Both APIs have 4 tiers: Free (tight limits for testing), Pro (recommended, best value), Ultra, and Mega.
What I'd Do Differently
- Start with documentation — I wrote it after building, but good docs should be planned alongside the API design
- Test through RapidAPI earlier — There were media type configuration issues that only showed up when testing through the marketplace
- Pick Python 3.12 from the start — Python 3.14 had dependency compatibility issues that cost me time
What's Next
- Adding Stripe for direct sales (better margins than RapidAPI's 25% cut)
- Building landing pages for both APIs
- Adding more endpoints based on user feedback
- Potentially wrapping an LLM for premium AI-powered endpoints
If you want to try either API, both have free tiers:
- TextPulse: https://rapidapi.com/gagebilliot20/api/textpulse
- ShieldCheck: https://rapidapi.com/gagebilliot20/api/shieldcheck