Senior Full Stack Engineer Contractor Who are OnBuy?OnBuy are an online marketplace who are on a mission of being the best choice for every customer, everywhere.We have recently been named one of the UK's fastest-growing tech companies in the Sunday Times 100 Tech list.All achievements we are very proud of, but we don't let that go to our head. We are all laser focused on our mission and understand the huge joint effort ahead of us needed to succeed.Working at OnBuy:. This is a critical role responsible for building, scaling, and evolving OnBuy’s core commerce platform across both front-end and back-end systems. As a Senior Full Stack Engineer, you will own engineering delivery end-to-end; from customer-facing experiences to underlying services and data systems. You will operate within a cross-functional team and be accountable for shipping high-quality, scalable solutions that directly impact GMV, seller performance, and customer experience. This role moves beyond “front-end vs back-end” you are an Engineer responsible for outcomes, not layers.End-to-End Engineering OwnershipDesign, build, and maintain full-stack solutions across UI, APIs, and backend systems Translate product requirements, designs, and wireframes into scalable, production-ready systems Own features from concept → development → testing → release → monitoring Front-End ExcellenceBuild responsive, high-performance user interfaces using modern JavaScript frameworks Translate UI/UX designs into clean, maintainable code ·Optimise applications for speed, scalability, and usability Back-End & Platform EngineeringDesign and develop scalable services and APIs Work across monolith and microservices architecture (Laravel / PHP services)Maintain and evolve legacy systems while building modern, scalable architecture Quality & Engineering StandardsOwn quality to production — testing is embedded, not a phase Write and maintain automated tests (unit, integration, end-to-end) Participate in and lead code reviews to ensure high engineering standards Debug, troubleshoot, and resolve issues across the stack Performance & ScalabilityOptimise systems for performance, reliability, and scale Work with large-scale data systems (MySQL, NoSQL, search platforms) Identify bottlenecks and proactively improve system efficiency Collaboration & LeadershipWork in cross-functional teams with Product, Design, QA, and other Engineers Mentor and support less experienced engineers Communicate technical concepts clearly to non-technical stakeholdersEssentialStrong full stack engineering experience, owning end-to-end delivery Proven backend expertise in PHP (Laravel) Strong JavaScript (ES6+) skills with modern frameworks (React/Vue/Angular) Experience designing and building RESTful APIs and scalable systems Experience with MySQL and NoSQL databases (e.g. Couchbase, MongoDB) Strong understanding of system architecture and microservices Experience with event-driven systems (e.g. RabbitMQ, Pub/Sub) and cloud environments (GCP) Familiarity with Docker and CI/CD pipelines Strong engineering practices: testing, code quality, and production ownership DesirableExperience in eCommerce or marketplace platforms Cloud experience (GCP) Experience with NoSQL (e.g. Couchbase) and Search platforms (e.g. Constructor & Solr) Experience working in microservices architecture This role is UK remote role you must be based in the UK and have full right to work in the UK. The day rate is £550-£650 per day. Please confirm your outside IR35 day rate upon application. This is a 4 month contract. Our CommitmentOnBuy is an equal opportunities employer. We are dedicated to creating a fair and transparent workforce, starting with a recruitment process that does not discriminate on the basis of gender, sexual orientation, marital or civil partnership status, pregnancy or maternity, gender reassignment, race, colour, nationality, ethnic or national origin, religion or belief, disability, or age. Just a heads up — we never use WhatsApp or any messaging apps to contact candidates. If someone reaches out this way, it isn’t us and let the recruitment team know. Who are OnBuy?OnBuy are an online marketplace who are on a mission of being the best choice for every customer, everywhere.We have recently been named one of the UK's fastest-growing tech companies in the Sunday Times 100 Tech list.All achievements we are very proud of, but we don't let that go to our head. We are all laser focused on our mission and understand the huge joint effort ahead of us needed to succeed.Working at OnBuy:. This is a critical role responsible for building, scaling, and evolving OnBuy’s core commerce platform across both front-end and back-end systems. As a Senior Full Stack Engineer, you will own engineering delivery end-to-end; from customer-facing experiences to underlying services and data systems. You will operate within a cross-functional team and be accountable for shipping high-quality, scalable solutions that directly impact GMV, seller performance, and customer experience. This role moves beyond “front-end vs back-end” you are an Engineer responsible for outcomes, not layers.End-to-End Engineering OwnershipDesign, build, and maintain full-stack solutions across UI, APIs, and backend systems Translate product requirements, designs, and wireframes into scalable, production-ready systems Own features from concept → development → testing → release → monitoring Front-End ExcellenceBuild responsive, high-performance user interfaces using modern JavaScript frameworks Translate UI/UX designs into clean, maintainable code ·Optimise applications for speed, scalability, and usability Back-End & Platform EngineeringDesign and develop scalable services and APIs Work across monolith and microservices architecture (Laravel / PHP services)Maintain and evolve legacy systems while building modern, scalable architecture Quality & Engineering StandardsOwn quality to production — testing is embedded, not a phase Write and maint