I agree your money your business but it is quite a big mistake to lose that much money. You bought 5k worth of stuff and sold it on cheaper. Where's that money? There must have been some return. Also, where did the money originally come from? For your parents to borrow money feom you and know there is money in the thoisands in your account sounds as though they're either exploitative or the opposite and play a supportive role with some understanding between that the money is there for a certain purpose. If you're a minor and the money came from your parents and the bank account is a step into adulthood and independence then this might be a situation where lying is wrong. That is to say maybe it isn't a case of your money your business. It's clear we don't have all the info so any advice given is prone to being entirely out of context. Part of me feels maybe this suits you.
II know parents in Korea can be quite domineering. It's a different world to the US in parenting styles. I would say be honest and learn from your mistake, losing so much money. Making your account independent of them is fine but make sure you're ready to go out on your own without making such rudimentary errors with your finances. It may be that you need further support feom your parents so no sense alienating them prematurely.