Buybacks should be with the set they were inserted in. It can't possibly be listed with a set from 1989 because the buyback was not available in 1989. The Database has a long-standing tradition of listing cards where they were found in packs, even if it's from a different year. Look at the modern SP Authentic hockey sets, the updates from previous years are listed with SP, not which set is being updated.
A complete set is probably not realistic, and nobody knows how many and what cards were stamped. (Some people have even pulled DONRUSS cards with a Topps stamp!) The best way to do it is to document them as they are found, with the understanding that there could be thousands upon thousands of different options.
I don't do baseball but I have pulled several buybacks in my topics. 4 if my memory is correct. One of them was signed and hand numbered to 3 copies, of one of my top 5 favorite players. That was a good day.
For the people I've seen collecting buybacks, the most popular way to do it is to do a numerical set- one buyback of every card number, regardless of what year the original card was issued or when it was stamped. I know several bloggers are doing this, at least three. Some people are also collecting them based on whatever host set it is stamped. Bloggers Night Owl is doing 1975 and Shoebox Legends is doing 1990. Others chase players or teams. Surprisingly I have NOT seen anyone building a one buyback per year set, possibly because it would be too easy to accomplish.
Buybacks do not appear to exist for any sets pre-1957. The older cards would not fit into the modern packs as the cards are smaller now.
VERY slow trading due to health problems. Not transferrable so safe to trade with, just moving is painful and can't always access the cards.
Cardboard History My COMC
New Collection Website: Cardboard History Gallery (Still under construction)
Tips on how to make your scans look like the card does in hand (No more washed out, fuzzy scans!):