But clothes aren't perishable, so why would you only be able to sell 500, rather than it just taking twice as long to sell all 1000?
Because it won't take twice as long, but 10x as long. There's typically a large rush on a new design, followed by a slow tick in sales. Meanwhile you have to pay to warehouse it, pay tax on the inventory, etc.
Fashionable clothes are perishable. "Nobody" wants to buy clothes from last season or last year.
Storing the clothes until they come back in fashion is expensive... and some materials really won't be useful after sitting for 10 years anyway. (Elastic bands really are perishable)