You don’t need to replace it yet — most of these stains can be lifted unless they’re etched into the plastic. Based on what you described (yellow/brown underside on plastic/melamine), here’s a realistic, step-by-step “try this before replacing” plan:

Stains look shadowy or “inside” the plastic
Surface feels rough or worn
Color doesn’t change at all
👉 It’s etched into the material — cleaning won’t fully fix it

💡 Honest verdict
70% of cases → peroxide + baking soda fixes it
20% → Magic eraser finishes the job
10% → permanently stained → replace
🧠 Practical advice
Toilet seats are cheap (often $15–30), so:

Try the peroxide method first
If not mostly gone → don’t waste more time, replace
If you want, I can give you:

The exact strongest product you can buy locally
Or a fast 5-minute “good enough” cleaning hack (for rentals / quick fix)