You should use prices because the following verb you used is plural - were. This means you are using two prices - the price of hardware and building material and the price of IT products.
"As the selling prices of hardware, building materials as well as IT products were under downward pressure due to stiff competition, the company's business environment is expected to be remain challenging in the coming few quarters."
Note that you can use plural prices for single entity there because that entity itself contains products. For instance, The prices of IT products --considering several products' prices.
The word price is countable and uncountable both.
You may consider a better version of the sentence as suggested by Jay and Damkerng: As the selling prices of hardware and building materials, as well as IT products, were under downward...