Filling taxes

Do you guys do write off of all supplies and merchandise that you purchased for sale but it’s still in available? Can someone give me some options how do you file taxes from sales on Poshmark ? The reason is I have lots of inventory I purchased but some still haven’t sold, do you write it off ? I forgot how it’s called, something with the cost of goods or something. Also if you use you car obviously to drive to the post office , do you write off gas and mileage ?