Is this the first time you're reselling or have you done it before? Because the answer depends a bit on the type of ticket and how urgent you are.
I agree that platforms like Ticketswap and Vibee are safe, but I think people sometimes don't really understand how they work. Yes, the fees are high (usually 10-15% or more), but their system protects both sides: the money gets locked until the buyer receives the ticket and confirms. That's really important because it stops someone from taking your money and then saying they never got anything. The bigger risk is really losing a good chunk of the original price to commissions, not so much getting scammed.
If the fee seems too expensive to you, you've got other options, but with more risk. There are people who resell directly in Facebook groups or Reddit (like r/Portugal or fan groups for the artist), and then you can negotiate the commission or there's none at all. But you've got to be careful there: bank transfers can be reversed, the ticket could be fake, etc. If you go that route, at least ask them to confirm they've bought it before you give them the entry, and if possible do the transfer in a way you could prove later (it's not guaranteed, but at least it's on record).