I built an app (PreSale) that generates secondhand marketplace listings from a brief text input or photo. The interesting technical bit isn’t the generation itself. It’s how I made the output actually useful rather than generic. The problem with general-purpose AI for this task If you ask a standard LLM to write a Vinted listing for a “blue Zara dress,” you get a technically fine description but the pricing is a guess and the format doesn’t match how real sellers write. It’s the difference between “plausible” and “useful.” What I did instead I collected and analysed 10,000+ real secondhand listings, then built a structured pricing algorithm from the patterns: Mapped brands into tiers (Low/Medium/High/Super/Designer) with distinct pricing ranges Categorised item types with base price ranges that reflect actual sold data Quantified the impact of condition, descriptors (vintage, cashmere, limited edition), and other signals on price For non-clothing items, I integrated real-time eBay sold data as a pricing source This all gets encoded into a detailed system prompt that constrains the model’s output. The AI handles natural language generation and flexible input parsing, while the pricing algorithm provides the domain-specific accuracy. Photo recognition I also added a vision model (Qwen3-VL) that can identify items from photos (brand, type, condition, colour) and feed that into the same pipeline. The challenge was getting it to reliably distinguish between similar items (e.g., a jumper vs a cardigan) and correctly identify brands from logos or labels in photos. Results The generated prices are now close enough to real market value that most users accept them without editing, which was the goal. The descriptions match the tone and structure of high-performing real listings. Curious if others here have worked on similar “domain-specific AI” problems where the challenge isn’t the model capability but encoding the right domain knowledge into the system. What approaches worked for you? App Store link: https://apps.apple.com/gb/app/presale/id6759057439 submitted by /u/Lili_thepink
Originally posted by u/Lili_thepink on r/ArtificialInteligence