Blue and red spots on my iPad screen

Stuck or dead display pixels are unusual, but not unknown.

The only remedy for pixel defects, should you choose to seek repair, is complete replacement of the screen assembly - this being the single most expensive component of your iPad.

Apple offers a fixed-price repair for qualifying devices. For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair will be expensive - but somewhat less than the cost of a replacement iPad.

You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

Repair can also be arranged at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) - where your iPad can also be assessed prior to committing to repair:

Genius Bar Reservation and Apple Support Options - Apple

Could be bad pixels on the display but you would ultimately need to have it inspected to know for sure. The question becomes if the person that sold it already knew issues were there or starting. Ultimately it’s up to you if you want to pay for a repair which if you do it the safest route via Apple or an authorized repair facility it may be much higher than you are looking to spend. Get it looked at by Apple to know what your options are and then decide from there.