Full Stack Engineer (AI Marketplace, Messaging, Geo) — Founding Role

Full Stack Engineer (AI Marketplace, Messaging, Geo) — Founding Role Costmar, Inc Hallandale Beach, FL Job Details Part-time | Full-time | Contract $64,578.65 - $77,772.13 a year 23 hours ago Qualifications Authentication Node.js Database systems development Full-stack development OAuth 2.0 Real-time systems Online marketplace sourcing Production systems Express.js PostgreSQL Next.js Developing and maintaining backend systems UI Back-end integration AI Geocoding Full Job Description About Costmar Costmar is building a next-generation AI-driven marketplace platform (web + mobile) that redefines how buyers and sellers interact. This is not a basic marketplace or CRUD app.

The platform includes:

Real-time messaging between users Geo-based listings and search (maps, radius filtering) AI-assisted listing creation and matching Scalable backend architecture We are looking for a strong full-stack engineer who can take ownership of the system and grow into a technical lead. Current State Core platform is already in development UI and initial codebase are partially built Architecture is in progress and open for improvement Your role will include: Reviewing and improving existing code Completing and scaling core systems Making key architectural decisions

What You'll Build:

Marketplace core (users, listings, workflows) Real-time messaging system (chat, notifications, events) Geo/location features (maps, distance search, filtering) AI-powered features (Open

AI / LLM

integrations) Scalable APIs and database structure Deployment pipelines and performance optimization

Required Skills:

3+ years full-stack development experience Strong React + Next.js (SSR) experience Strong Node.js + Express backend experience Experience with PostgreSQL Experience building production APIs Experience with authentication (JWT, OAuth) Experience with file uploads / media handling

Preferred Qualifications:

Experience building marketplace platforms Experience with real-time systems (WebSockets, chat, events) Experience with maps / geolocation (Leaflet, Google Maps, etc.) Experience with AI/LLM integrations (OpenAI, RAG, etc.) Experience with AWS / Vercel /

Docker What We're Looking For:

Ability to design systems, not just implement features Comfortable working with and improving an existing codebase Strong problem-solving and ownership mindset Ability to move fast and iterate Interest in growing into a technical leadership role Not a

Fit If:

Only experience is landing pages or simple dashboards No backend/API experience Not comfortable working on complex systems Prefer strictly defined tasks instead of ownership Compensation $65,000 - $80,000 per year (based on experience) Contract / part-time initially Opportunity to grow into long-term core team role How to Apply Please include: Resume GitHub or portfolio Brief description of a system you built (preferably marketplace)

Pay:

$64,578.65 - $77,772.13 per year Application Question(s): Describe a system you built that includes either: (1) real-time messaging OR (2) geolocation (maps, distance search) OR (3) marketplace or similar B2C C2C platforms Include technologies used and your role.

Work Location:

Hybrid remote in Hallandale, FL 33009