Ahsan Allahu ilaykum. Is it permissible to private message with the opposite gender through a platform that is dedicated to buying/selling? That's the only reason for messages, and it's required to buy/sell. If relevant, there's no indication of my (female) gender on my profile

Ustadh AbdulAziz al-Haqqan

Yes, as long as you are interacting with them for the business, the transaction, and nothing more