Most bike manufacturers have an online archive that goes back several years and there is always Bikepedia for tying bikes to a model year. Component mix is usually a better way to id the year. Finding a bike with the right components and good fit is far more important than having the model year correct. Back in late 1995 I found an outlet store that stocked a full line of carbon and aluminum framed Cadex bikes in the right size. I really did not know the model year but assumed the bikes were overstock from 1994 or 1995. Being a dyed in the wool retro grouch I went for the carbon framed CFR 3 with 105 gruppo and downtube shifters rather than the then new STI on the other models. Seventeen years later Bikepedia told me it was a 1993 model. At less than 50% of msrp it was a deal, but most important it was set up the way I wanted. Model year was and is irrelevant.

I agree most sellers are probably like me and they remember the year the bike was bought but probably don't know the model year.