It is an early/mid nineties commuter/hybrid frame. Neatly made with triple butted spiral spline cromo. Same for the forks. Good stuff.
Koga Miyata is still in business. Their frames come from Hodaka since mid/late nineties. Before that from Miyata Jp. Koga always served the higher ends of the markets. Nowadays they're also the luxury division of Miyata in Japan.
They have a pretty advanced online catalogue showing for example bonded aluminium bikes. Don't know whether the frames are any good, but they look neat. Online catalogue =>
That frame from Bikeman is a Dutch Miyata, called Koga-Miyata. Available on almost every corner in Holland. Nothing special in my opinion. They are heavy but very durable and nicely designed frames. They make a lot of touring bikes and randonneurs and have a lot of experience with that sort of thing. It's not worth much, but then again: the asking price is pretty sharp. Could be turned into a durable cummuter bike.
It is not heavy. My parents have a frame like that build up with lights, fenders, rack, Brooks seat, bell etc and both are only around 15,5kg, what I consider not much. My parents' bikes are from 1992.