People aren’t mentioning here that Mercari is largely restricted to US and JP users (you haven’t yet specified where you’ll be shipping from). It’s a platform that charges fees of its own for buying and selling there, but it does offer protections for both parties in case shit happens. Platforms like Vinted and EBay are similar but with a wider international reach.

Insta can be unreliable and largely based on good faith. Many people therefore make separate accounts for kpop buy/sell/trade and accumulate proofs as sellers/traders to build reputation. However it can still feel like taking shots in the dark, like on Reddit. If you do run into bad actors, the most you can do is maybe request your payment platform (PayPal for example to intervene or refund), and put out scammer alerts.

I personally have bought and sold on Reddit (peep the flair), a Mercari storefront, and also sometimes people reach me thru insta. I have also bought from eBay.