Yes, thank you for the tip. I just came back yesterday night from Leipzig where I attended three exciting performances of the first three Wagner's operas : Liebensverbot, Die Feen and Rienzi.
Oelsner is a nice shop with friendly staff. The antiquariät section is at the first floor, wide enough to accommodate a grand piano and small concerts.
Regarding the used sheet music, it is properly classified by instrument. The solo piano section unfortunately was rather poor and I did not find anything really exciting, althouh I bought a few baroque pieces that I probably already have in other editions.
Anyway, it was interesting to visit, and I might be luckier if I return another time.
There is also a Antiquariät bookshop near Skt-Nikolai Church which has an interesting music section, mostly German books of course and God knows how abundantly and well they did write on music, with a few bounded scores.
Oelsner is still run by Michael Rosenthal, who has been the manager/owner since at least 1989, when the Berlin Wall came down. I was in Frankfurt for the music fair the following March, and it was a very poignant and emotional time for Germans, with hundreds, probably thousands of east German musicians coming to the West for the first time.