First generation iPad Mini 9.3.5 (13G36)

iPad2, iPad3 and iPad mini1 can only be updated to iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular models).

Apple ended update support for these models in September 2016. Your iPad cannot be updated to iOS 10 or later major versions of iOS, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. This limitation cannot be bypassed; these are 32-bit architecture devices, whilst all recent versions of iOS/iPadOS are designed for 64-bit hardware platforms. iPad2, iPad3 and iPad mini1 are obsolete; as such, Apple do not accept them for trade-in.

As for private sale, being very limited in capabilities and availability of Apps, they are similarly unattractive for purchase by others.

You can sell it privately.

Or you can try to Trade It in to Apple for a gift card, not sure they still take that model in for Trade In however. Link-> Apple Trade In - Apple

If they don't take it in for Trade In anymore, they will recycle it for you.

Or you can keep it, and repurpose it as a digital picture frame.

Just some suggestions.