Usage patterns
買いが入るfor buy orders/purchases to come in (esp. in trading)
Buy orders are coming in for this stock.
買いを入れるto place a buy order
I placed a buy order when the stock price dropped.
買いが少ないthere are few buyers (weak demand)
There are few buyers for this product.
買いに走るto rush to buy something (often out of fear of shortage)