Bonsoir people!
After I found out about the scam, I started looking on Gumtree and I found some advice (why didn't I look there BEFORE I did anything??) and a forum where users share their "scam experience". What I found there is quite scary, and my offer was definitively a dirty ol' scam!
So let me share my experience! How to avoid a scam? (most information comes from Gumtree)
1) The best way is to actually visit the flat/house you covet. It's the best way to check if your landlord is honest and if the property is genuine. Remember that it is highly advised not to visit a flat alone. Bring a friend with you; it's safer, you never know!
However, it's not always possible. It wasn't for me. So what to do in that case?
2) If you know someone who can visit the place for you, ask them!
3) Do NOT send a copy of your ID (passport, identity card, driving license, or anything)! The scammer would be able to use it for another scam. It is dangerous.
4) NEVER provide your bank details to someone you do not know.
5) Use common sense; if something seems too good to be true, then it probably is!
6) If your landlord wants you to transfer money (such as the security deposit) through Western Union, or something similar, then it's probably fraudulent. It often happens that the scammer asks their victim to transfer some money to a friend to make sure they can actually pay, and then intercept the money thanks to the scanned receipt the patsy would have sent.
7) If your landlord tells you they are currently abroad/oversea, and thus cannot meet you in person to make you visit the flat, then they are most certainly a scammer. They will use excuses such as: "I have found a new job in [insert name of city here]".
8) It often happens that scammers cannot provide a phone number.
9) It is now known that many scammers come from Africa (Nigeria). Thus if you think your landlord's English is a bit awkward, beware!
10) Your supposedly landlord will make everything to make themselves trustworthy. That happened to me. My landlady told me I could pay for the extra weeks whenever I wanted to. She told me I could deduct the transfer charges from the security deposit. Stuff like that.
11) Check the pictures, if there are any (and if there are none, ask for some). I have found a website that could be really useful and which allows you to see if a picture can be found somewhere on the Internet: TinEye.
The Gumtree team made a nice video on how to stay safe online. I found it useful. If only I had seen it before I got scammed! It basically sums up all what I've said.
If you have any more ideas, please comment, and I'll add your advice on here =)
To finish with, here is a reportage I've found on the BBC News website. May it help you!