What’s the worth of a sea turtle?

Trying to figure out so I can make a good offer for one. I tried using a value website and created what I thought would be a fine/fair trade for it (alicorn + an endangered egg) and no one seems interested when I try offering that for their sea turtle. Should I add more to my offer? Or should I change it?

I’m not very good when it comes to trading and values, but I’m trying to learn so I can get pets that I want without massively overpaying or accidentally lowballing someone.

Also how do I learn demand? I know that might play a factor but I’m genuinely so confused when it comes to value and demand

Thank you to any and everyone who comments!! I greatly appreciate it