Software to Fetch Market Data to Calculate Price, need help please!

I am looking to build an ecommerce site which buys used electronics, however, I face a problem because the products are old we ask the customer a series of question based on the size, condition, accessories included, etc. do predict a price for the item. After they enter the questions a price calculator gives the customer an offer that they can accept or deny. I know it is possible to manually enter the prices but for a large database this would be very time consuming. Is there any type of API or software that can calculate prices based on the product's condition, longevity (depreciation), supply and demand of economic trends, so basically the market price for retailers. I know this website gazelle.com uses it but how? Any ideas would be greatly appreciated and I will pay money to have this project completed, thanks. Also if you think I am new and incredible please check me out on digital point forums with the same username I have a good reputation so I am serious about this.