The price of a used vehicle can vary based on several factors such as location, condition, demand, and the overall market. To determine a fair price for a 2018 Yamaha XMAX 300C with an unknown service record and 28,937 miles, it would be advisable to consult local dealerships, check online marketplaces, or consider getting a professional valuation from a trusted mechanic or vehicle appraiser. These sources can provide you with more accurate and up-to-date information to help you make an informed decision regarding the price.