iPhoneTools.io provide Free IMEI Blacklist check tool. We deliver the result automatically within a minute and you have 5 free checks / day. The limitation is about to avoid spammy requests.
It's important to check Backlist (ESN) Status before buying an Apple device online (like from eBay) because the handset may not work with any network if it's Blacklisted.
This tool is also great for you if you're the seller. You can send our website link as a proof of ESN to your buyers. Example: http://iphonetools.io/check/blacklist/000000000000000