Yeah the supply and demand thing is key here. People in developing countries are exploited because there is a financial incentive to exploit them. That financial incentive comes from people buying their products directly, because that is how they generate revenue. But there is no way they can generate revenue if you only buy their products second-hand, so its not incentivizing them to maintain their sweatshops or whatever.

The most damage buying second-hand can do is by generating revenue for these companies indirectly. For example if OP was in position where wearing these items contributes to fashion trends that cause others to buy the items directly. But unless OP is an Instagram influencer it something (which is doubtful, given that they’re on lemmy) then these indirect, second-order effects are very unpredictable and hard to measure.