Bear in mind at all times that web sites are public places.
Do not meet with a stranger unless you are in a public place and with people you know and trust, AND you have contacted them via a number which you can ascertain for yourself is not a mobile phone number and is either the number of their home (by landline) or their work (by landline.) Give these numbers to a friend and let the person who you are meeting know that you have done this. On the first meeting, making sure it's obvious you've let someone else know where you are, and who you are with (by mobile phone) is a good move.
You shouldn't assume on this site (or any other) that because of the nature or content of what is posted on it, that it feels 'safe' and that therefor you could 'rely' on it in the same way you could do so with a group of people that you already know. You very probably don't know anyone who is posting on this site, and they are therefore total strangers that might pass you in the street.
If you visit a private dwelling (for instance to look at something for sale) then it is far better to go with a friend. The above precations with respect to phone numbers should also be followed.
If you come across something on this site which just isn't right for some reason, then 'reply' to the posting in the usual way, saying what is wrong. The moderator for that part of the site will almost certainly then remove the posting and watch out for future occurrences.