Its a fairly simple answer.

The East Germans had massive stocks of materiel and supplies, and during the cold war, the majority of its army units were kept on high alert. It had the most tanks per capita of any country in the world as well, Fielding some 2200 T-55's (in multiple variants) and approx 300 T-72's on a total population of some 16 million.

When the wall fell, and with the unification, the BD evaluated a lot of equipment, which ranged from obsolete to quite useful. For instance, the BMP series of armored fighting vehicles were tested and were found to have serious issues (first being they were stuffed to the brim with Asbestos) but were otherwise worth retaining. The problem was that the former East German military heavily objected to this after being allowed to play around with the German Marder series so the idea fell through. The only piece of kit the Germans retained was the LSKs Mig 29 fighters, which were rather capable and had the added benefit of being usable in DACT.

The rest was either returned to Russia or sold off on the cheap. For instance, the Finns bought a lot of the DDR's T-72's which were well maintained and quite new.