Businesses that sell used bikes and/or parts clearly have value for communities and the environment. They are a way of reducing waste and reallocating gear that still has useful life, they are a resource for DIY-minded cyclists or tinkerers, they serve populations that might be priced out of mainstream retail shops, and the list goes on. However, they need to get their product from somewhere, and bicycle theft is a major problem in many of the same areas where the legitimate used market is also naturally strong.
If you want to operate such a business and keep a clear conscience, what do you do?
Depending on where you are in the world, there may be various law enforcement agencies to run a serial number with, or independent websites like bikeindex.org to check, but they threaten to be token efforts, as many thefts go unreported. And in the case of the non-LE sites and databases, experience (mine) has shown that the situation can become murky and time-consuming when you do find that the bike in front of you is listed on one, since local law enforcement may not be willing to simply take control of the situation at that point in the same way they would if the bike was officially reported stolen to them.
Parts are their own challenge. Some number of legitimate users of your business will have interest in supplying you with parts for trade or sale, but taking these transactions clearly risks becoming an unintentional fence for thieves. Relying on heuristic discretion (this looks hot, that person looks sketchy) may seem like an obvious answer, but allows bias into the process, is difficult to gauge the effectiveness of, and leads to stressful situations in practice. Reselling used parts can seem impossible to do ethically, which is unfortunate because it is essentially a form of recycling.
What proven models exist that can solve or reasonably mitigate these problems?