i'm having locked iCloud issues. I bought a used iPhone 5s.

There is nothing anyone here can do to help you. That seller will need to recover their AppleID and then disable activation lock for you.

Show them this - https://support.apple.com/en-us/HT201351

and this - If you forgot your Apple ID - Apple Support

Without them knowing the password for their AppleID, there will be no way to remove activation lock, and thus no way for you to ever use that phone. If they cannot do that, they should give you your money back.

If the person who sold it to you is the original owner and purchased it from Apple, they are going to have to take it back to Apple with the documentation and Apple will unlock it for them.

Personally I don't see that as likely because the iPhone is probably stolen and hence Apple will not do that for them. In which case, it will never be usable by you.

Hi

The person who sold you the iPhone need to go to iforgot.apple.com

To get this information if he cannot do this get your money back.

Cheers

Brian 🇬-1F1E7;