Based on the brief description and poor picture, it appears to be a circa 1976-1977, Sekine RM20. These bicycles used a straight gauge CrMo main frame with hi-tensile stays and forks. The only Shimano 600 is the shifting system and it is the first generation, not the later arabesque 600EX. The crankset should be a swaged, cotterless, SR design. On the plus side, it should have aluminum, 27" rims with quick release hubs. For its era, it was a nice bicycle on the cusp of entry and mid-range. The workmanship and paint was typically very good, but the downtube decals often fall off. Expect about 24-26 lbs and a soft, comfortable, but somewhat dead ride characteristic. Whether it is worth the money will depend on the local market conditions, the actual mechanical and cosmetic condition, and the fit.