During the movie we see Robert, a regular guy who likes to be neat. We find out that he is a retired bad-ass of some type.
At the end of the movie, we see him back in the diner, but this time he is wearing different style of clothing, mostly black in color and has a look of purpose on his face. Then we see him on what looks like craigslist, responding to a post for help.
Does this mean Robert decided to come out of retirement (possibly for a sequel)?
Yes, but not officially. Whatever official previous job McCall had (as some kind of spyssassin, likely for the government), it is unlikely that he'll continue to do that. However, seeing that imagery with him checking people that need help, it is very likely that he'll continue to inofficially help private persons that need help in unordinary circumstances.
In addition to the hints you already provide in the question, this is also evidenced by the fact that the movie is actually based on a TV-show of the same name from the 80s. While I have not seen that, it seems to put McCall in exactly that position as a travelling vigilante who helps innocents. So it is very likely that the movie understands itself as some kind of prequel-in-spirit to the TV-show, prompting McCall to actually start his inofficial carreer as a vigilante.
Wikipedia also supports this idea in its plot summary, as well as the possiblity for a sequel:
...McCall is inspired to continue using his skills to help people in need and posts an online ad, identifying himself as "The Equalizer". He soon receives another plea for help and agrees to answer it.
...
On February 24, 2014, it was announced that Sony Pictures and Escape Artists are planning a sequel to the film, with Richard Wenk penning the script. In early October 2014, Fuqua said in an interview that there would be a sequel to the film only if audiences and Denzel wanted it. He said it was an interesting character and the sequel could have more of an international flavor.
