Cannot give you an accurate answer just that eBay messages have always been short on clarity, but ' unusual activity' usually relates to an expensive item being sold, which is not the norm for your account.
My bet would be the audio equipment which sold for £2995 , flagged this message up for you.
Contacting Customer Services to speak to a human has been an issue of late, using the link below some have had success contacting CS first thing on a weekday morning at 8 a.m. and using the app? As said it seems hit and miss to get a live agent, if it is the chatbot keep entering wish to speak to agent, that too sometimes works.
https://www.ebay.co.uk/help/eua?id=5190
@moorgate-acoustics