eBay has 1pcs STM32F407VET6 for $4.80+$1.50 shippping = $6.30 shipped. $6.30 is below $6.46
gigadevice happens to have https://www.gigadevice.com/products/microcontrollers/gd32/arm-cortex-m4/high-performance-line/gd32f407-series/gd32f407vet6/ product.
Is there a definitive objective technological test to know which one you have I wonder, or is the only way to buy from a reputable vendor?
I've just posted same question at st.com https://community.st.com/s/question/0D50X0000B0xX6JSQU/singe-stm32f407vet6-shipped-for-630-is-it-real