This is useful not only if you are buying a second-hand phone to check that the IMEI matches the actual description of the phone (it's not fake) but also if it was reported lost or stolen or is blacklisted (or blocklisted now). I'm not sure how widely the site is retrieving the info but it certainly matched my phones perfectly.
I also managed to report one phone as stolen and another as lost (which were the cases) so it can be useful to flag this here too if this happens to you. There are actually many sites which provide these services.
A way to quickly check a phone's IMEI number is to dial *#06#
which will send a USSD query and return the IMEI number on the screen.
See IMEI CHECK - Free Online IMEI Number Checker | IMEI.info
Check IMEI Number and find out hidden info. Check hardware specification, warranty or BLACKLIST status and more for FREE. Over 110 mln checked IMEI's in our database.