This is will be a very short guide.
It does matters sometimes... However, this is a grey area. If dude A haven't scammed dude B, that doesn't mean he/she will not scam you. People are unpredictable sometimes.
GET TO KNOW THEM
Get to know the person you're trading with. I personally don't do business with people I know nothing about.
There's probably a lower chance that a scammer will share personal information with you.
Try to get as much information as possible from them - where are they from, what else are they selling, when did they start it, with who else was he/she; is he/she working with. Check for common friends or business partners/L2 servers.
LEARN HOW A TRADE WORKS
If you're buying L2 services and you're the buyer - 1st test what you've asked for, 2nd pay for it and 3rd get the product.
If it's about trading items in L2, I'd personally push for the items first before money - in-game items are less valuable. Would be nice for the buyer to provide a proof that he/she has the money.
This is perhaps the best option to go with. Only use very trusted persons that are doing this for a long time.
If you're trading items in-game - this is very easy and it needs no explanation.
If you're using a middleman for L2 services, chances are the middleman will not delete the code/product he got from the seller after he trade. If you care so much about this as seller, then use a middleman which has less interest for L2 scene. Same rule apply to other services.
You can try to have a voice-chat with the person you're trading with. Hearing each other is the most human thing you can do. Don't you think?
Be aware! Scammers are mostly rushing, want to have it done as soon as possible.
Is the person you wanna trade with suspicious? Abort it! There are other persons you can trade with. He/she is not the only one.