It depends on where you are. People from Changping County usually go to Shuitun Wholesale Market. Seems like taking bus 5 to get there. If you are closer to Huilongguan, there is the Huilongguan Agricultural and Secondary Products Wholesale Market, which is very large and has a wide range of products.
If you are in Sunshine, take 919 and get off at Shuitun. A little further west is the wholesale market. If you go to Huilongguan, you can also take 919 to Shahe and take local train 345 to Huilongguan. However, it is on the east side of the road and it is not convenient to drive there. drive or ride a motorcycle. There are more things there than Shuitun.