For used auto parts, the main problems are electronic failure, which is extremely difficult to test, and metal or plastic wear. Wear can be hidden inside the part, and it's not always possible to disassemble it to see what's inside. Counterfeit parts (made in China) can also be problematic. For example, the metal may be different, not the correct one.
Here are some ideas for making the used auto parts purchasing service more convenient:There shouldn't be a two-week return policy for used parts, as it's difficult to properly test the performance of used parts, and used parts themselves are easily swapped (you can't swap a new, factory-issued part for a used one, and you can easily swap one used part for another). Nowadays, people often buy parts from other regions, meaning they pay for shipping. Return shipping would be a significant expense (auto parts are typically large and heavy).
The price of a used spare part should be set by an auction and a system of genuine reviews similar to the sale of services. This means that instead of services, the seller sells used parts without the option of returns (no claims). Since the range of spare parts is vast, it's necessary to enable a filter for only a specific part (brand) or category of parts. A review filter is needed: all reviews, only this section, and only this spare part.
Reviews must display the amount spent on non-refundable fees (fees for the review service, as well as shipping costs and tax) so that this data can be used to identify potential fake reviews (the "one negative review erases a certain number of positive ones" system should also work here).
In general, you can even do this without using an honest review system, because reviews for products with delivery (without the option of returns) are difficult to fake. Therefore, you simply need to indicate in each review the amount the buyer spent on both the product itself and the shipping costs (and attach receipts that can be tracked through postal services).
And most importantly, it's better for one seller to focus on selling one type of auto parts (for example, only headlights, or only wheels, or only steering wheels, etc.). This will make it much easier for buyers to monitor the quality of used goods (defect rate, percentage of working parts) through reviews, and therefore more accurately determine the value of a purchase at the stated price. All this will benefit such a business (increasing its sales).