Current owners’ ratings are the least objective. Bias is a real phenomenon. We pinheads are not immune.
Here is my somewhat tongue-in-cheek proposal for ratings adjustment:
-Owned a game, sold it, then rated it- minimal bias, no adjustment necessary
-Played a game a lot and rated prior to owning- minimal bias, no adjustment necessary
-Played a game a lot and rated after owning- small bias, reduce rating by 2%
-Bought a game with little to no prior playing experience then rated it- moderate bias, reduce rating by 5%
-Bought a game on preorder before anyone had even seen it, much less played it- maximum bias, reduce rating by 10%
These adjustments may be a bit conservative.