Spielart
Müllerstraße 39, 80469 München, GermanY

Our esteemed member Alsterstreek kindly provided me with the following information for when I was passing through Frankfurt recently :

Modellbahn-Rhein-Main, Max-Planck-Straße 18, 63322 Rödermark (https://www.modellbahn-rhein-main.de/) has a large selection of second-hand goods.
The shop is walking distance from “Rödermark-Ober Roden” station (just under 1 hour's travel time by S-Bahn from Frankfurt main station).
I (PJ) can vouch for this dealer since I have bought a fair number of second-hand items from him online. He has items of excellent quality
and a huge range'.

Darmstädter Modellbahnshop, Grafenstraße 14-16, 64283 Darmstadt (http://darmstaedter-modellbahnshop.de/) can be reached by public transport from Frankfurt Airport in 45 minutes.

MEDER, Berger Strasse 198, 60385 Frankfurt am Main (https://meder-frankfurt.mhi.de/) is located next to the Bornheim-Mitte subway station (line U4), 10 minutes from Frankfurt Central Station.

Regards,

PJ

Since my father-in-law was an Opel man, and parents in law live near "Opel-City", in the shadow of the airplanes landing on Frankfurt airport, I am often in the region.
My favourite shop is in Hofheim: https://www.bahnstrasse22.de/
last time I was there, there was less 2nd-hand-ware, but I found some fine things. You must know he accepts no card paying, so get some liquid money before entering the shop since the nearest money distributor is in the city, at several kilometers from there.
Some years ago, there was a Sparkasse in face of the pharmacy, but they closed the office and did not even let the automatic distributor.

If you consider Mainz in the area of Frankfurt, there is Theis https://theis-modellbahn-mainz.de/ two shops at some houses only from one to the other. I was there twice or maybe three times, but I am not charmed. High-pricing for 2nd hand and more interested in getting money than in being friendly. No comparison to Bieger in Hofheim.