There is a cluster of bike and scooter sellers, from street vendors to large stores, right by Metro Sokolniki. But go there only if you know you know your stuff. For reliable middle-of-the road bike I’d recommend one of several French-run Decathlon stores. They sell spectacular scrap too of course but they make no serious attempt to present it as something else.
The owner of www.velotransunion.ru runs a bike shop somewhere in the east end. You may want to talk to him. In previous years he was selling “value” bikes.
Free advice advice and assistance to those wanting to get a bike. It is subject to my availability but I’m pretty close to the Sokolniki cluster of shops and to one of the Decathlon stores.