The probability of crossing paths with a scammer is heavily tied to the kind of product you are buying or selling. Keep that in mind. For example, buying products for and from other parents like clothing or children books is usually pretty safe, while gadgets or gaming products are infested with people looking to take advantage of you.
Avoid giving your real name if possible.
Avoid giving your real address if posssible.
Avoid inviting people home if possible.
Never give your phone number or any other free information.
Use a secondary PayPal account, not your main one.
Make a basic plausibility check of the buyer/seller, make some google searches.
Minimize the risk as a buyer.
Never take the risk as a seller.
Do not deviate from this guide.
The goal when buying, is to attempt to let the seller take the risk of the transaction.
That's it. This is the only way to be safe. Go pick up the damn product, take a good look at it before paying, pay in cash.
This is how I buy anything worth more than 50€ (my personal limit on money I am willing to stupidly lose). It is a simple rule: Does it cost more than 50€? Then go pick it up. Not possible? then buy it in Amazon or somewhere safe.
It is really the only advice that works 100% of the time.
It turns out that, at least in some countries in Europe, anyone with your IBAN can easily take money using a feature called Direct Debit. That is enough reason for me to never give my IBAN to random anons in some app full of scammers.
As a buyer, Sicher Bezahlen is quite a safe bet. Sure, the risk is not zero, but generally speaking, if something goes wrong, Kleinanzeigen tends to give you as buyer preference if you request a refund. This is also one of the reasons why you won't find many sellers willing to use it either.
Just don't give any personal information like names or email adresses if you go this route. It lowers the chances of more complex scam methods like fake malicious emails.
Ask the seller for their email address and pay via Goods and Services (not Friends and Family). The problem is, you won't find any seller who is dumb enough to accept payment as goods and services...
Paying via Paypal Friends and Family is like sending an envelope with money to the seller. If something goes wrong there isn't any chance you will ever get your money back. I usually accept the risk for inexpensive items after doing some basic plausibility check.
The hard truth is that buying stuff from private people, unless done in person and paid in cash, is inherently unsafe: for the seller or for the buyer, one of them has to take the risk.
So sometimes it is not worth it. There are marketplaces like Amazon or eBay where you do have some protection even when buying used products. So my advice is to simply look somewhere else and maybe pay a bit more for the peace of mind.
The goal when selling, is to let the buyer take the risk of the transaction. Never take the risk as a seller. Never. You will eventually find some other buyer who is willing to take the risk for you.
But don't invite them home, well, unless you are selling your old bed or something big and heavy.
Ideally, for products that you can easily carry, let them meet you somewhere close to your home. I like to tell them to meet me in the closest gas station or supermarket and I use the chance to tank my car or buy some groceries.
If it is not possible for the buyer to pick it up and pay in cash, because you are selling to someone on the other side of the country, never accept any other type of payment other than PayPal Friends and Family.
Do not give them your IBAN.
Do not use "Sicher Bezahen" or any other payment method.
Do not accept payment via Paypal Goods and Services.
If they can't or don't want to pay via PayPal Friends and Family, say "no thanks" and move on.
Just make sure it was really sent as Friends and Family. There is no need to write your name or address on the package. Take pictures of the product while you pack it, after packed and at the post office. Take a picture of the receipt after you send it.
I am of course assuming that you are not scamming anyone and that you did everything honestly and correctly. Write a legally valid disclaimer for every product you sell so that it's clear you are not offering any kind of warranty.
If something goes wrong, it is bad luck for the buyer. It doesn't matter how friendly and charmant the buyer is. The best scammers are likeable people. Remember, you did everything right and honestly, you cannot know if the buyer acted the same way.
After all, you wanted the buyer to take the risk of the transaction.