How can I erase the old Apple ID and password?

You will NEVER be able to use this iPad. EVER. Activation Lock cannot be bypassed without the actual Apple ID and Password used to set it up. If you don't know it, the previous owner doesn't know it, it's a useless device. Apple will NOT help you. We CANNOT help you. So here are your options:

1. Return it for refund
2. If you can't get your money back, recycle it responsibly
3. Enjoy it as a paperweight or coaster

You can't bypass it, only the person whose account it's locked to can potentially help you

If it has activation lock contact the person you bought it from, only he/she can potentially help you

You can't: there is absolutely no way round this. Take it back to the person you bought it from and ask for a refund - he has sold you a completely useless device.