I collect omega’s for over 20 years, in the beginning there was way less info then there is today. So it came down to experience, watching ebay etc. During these days more and more lists are available online. But like many, I bought those links when they were dog cheap (516 always were know and relatively expensive) so now I have a pile of bracelets and endlinks and boxes and papers like many old time collectors hoard. Now it’s all dried up, and personally I only trade (say a 47 for a 640). A site that would sell them would have a hard time finding new stock and would be very expensive as with vintage supplies only will get rarer. Therefor buy what you can. There used to be USA and Australian sites, but they also dried up/ and are cut off by omega.