I think it is a really interesting idea. Would be nice to see how the “OPY”s for a certain watch vary over time. As more info is accumulated I guess you could split the major chrono types into sub-categories (probably linked to your “Guide to Chrono” categories?).

Would you try and track “OPY”s of known fakes/frankens as well? Possibly problem with grey area between franken and repaired.

There is probably someone out there in OTD land with a very comprehensive database of ebay auction results over a few years but maybe that is proprietary?