PayPal is the most popular just because it's easily available for everyone to use, though it is a high risk as it's somewhat easy for people to claim they have been scammed and PayPal will almost always go with the sender (buyer) unless you have solid proof backing you up - which most people have no clue how to do and end up being scammed. Refer to this to learn about PP chargebacks a little better.
Bitcoin is becoming more preferred as it's pretty secure and not easily possible to scam. Once a payment has gone through then that's that. People feel more safe dealing with Bitcoin as they don't have to really worry about chargebacks like they do with PayPal since transactions are normally irreversible. The downside to this is that Bitcoin requires a lot of verification to prove that you're you before you can buy/sell/trade any Bitcoin
Skrill isn't really that popular. It has a small spike prior to Bitcoin since a lot of people were charging back payments sent via PayPal, but then shortly died out since it's just annoying to use and it's as user friendly compared to the simplicity of PayPal. Skrill also has it's own little issues and sometimes has delayed sending time while PayPal is basically instantaneous.
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Unless I really know you, I won't accept PayPal from you over a certain amount. I don't trust too many people as I've been scammed a shitload in the past by all sorts, from Admins to no name scrubs I decided to give a chance. This is the internet and so people basically have nothing physical to lose should they scam you. Just take the necessary precautions to combat imposters and make sure you use a MM or really trust the person you're dealing with.