I want to buy a used phone. Can the seller to report the phone to blacklist after the sale? How to prevent this situation?

The important thing is to check that the phone hasn't been Activation Locked, because that would render it useless the next time you reset it upgraded the system.

Apple have provided a method of checking before purchase that an iOS device isn't Activation Locked:

http://www.idownloadblog.com/2014/10/01/activation-lock-status-check/

Basically you just go to https://www.icloud.com/activationlock/ and enter the IMEI or serial number and a catchpa.

Apart from that, get a proper receipt to prove you bought it from this person. Then if he reported it stolen it would demonstrably be attempted fraud.