Third, it doesn't mean that they will easily be led into believe anything the caster says. They only get advantage on rolls, but if there's never a roll(the outcome isn't in doubt), then they fail. You could almost certainly convince the charmed merchant to give you 10% off, but you would fail if you wanted him to give you stuff for free.
I never said that Being easily led to believing anything the caster says means "will do things out of character for the target."
If Maria the caster casts
charm person on Bob the merchant, Bob will have no immediate reason to be suspicious of Maria--the spell ensures that Bob views Maria as a decent enough person. If Maria tells Bob to give her something for free, then unless Bob is the type of merchant who normally gives or lends out stuff for free in the first place.
If Maria swears up and down that she'll give it back, she just needs it for a moment, and then makes a good Deception roll (made with advantage!),
then there's a pretty good chance that Bob will believe her. By which I mean, he will believe that Maria is being honest here, that she intends to bring back the thing. This assumes that Bob doesn't know Maria or has reason to believe she's a liar (assuming she is lying here).
Depending on Bob's personality, he may give her the item (believing he'll soon get it back), offer it in exchange for collateral ("I'll hold on to your bags while you do whatever it is you need the thing for"), offer it at a decent discount (which may be more than 10%), or simply say no ("I wish I could help, but I can't afford to have that item lost or damaged. I'm sure you'd try your best to keep it intact, but things happen"). What Bob
won't do is immediately say that Maria is a lying thief or something like that, because friendly acquaintances don't do that.
And this would be the case if Bob was a PC, too, because the spell made him see Maria as a friendly acquaintance. You, the DM, simply tell the PC that Maria certainly seems honest and sincere here. Hopefully the player--who should probably know they've been charmed--will be willing to play along or come up with a reason to deny the request to this friendly acquaintance. And this reason can be a complete lie, of course ("I can't give you this thing! It's my very favorite thing! My grandmother made it for me!"), but the PC shouldn't just attack, as PCs are wont to do. Because
in character, Maria seems honest and sincere.