The yoyo was shipped mint with the exception that I had completely forgotten that I had previously sanded the silicone recess so that it may better accept silicone. The damage described by OP is grossly exaggerated. That means it isn’t “mint.”
(also: how do you get DENTS in PLASTIC?) By knocking the yoyo into something hard.
Having shipped the yoyo for $30 (shipped), I tried to reason with the buyer, stating that I did not charge him shipping ($6). This is utterly irrelevant. He didn’t get what you told him he would receive.
That wasn’t good enough. You’re right. The way you handled this isn’t good enough.
I insured him that for what he got, (a near-mint, exceptionally well-playing Speeder 2), he made out like a bandit. Again, he didn’t get what you said he would receive. Period.
Still wasn’t good enough for him. That’s because it isn’t good enough.
I would have been happy to compensate the buyer, but I quite literally could not afford it, having already spent the money to box and ship the yoyo. This just makes it sound like you know you did wrong. You did. Save up some money and compensate him.
So I made $24 in pocket off a near-mint $55 yoyo. The price you sell your yoyos for is on you.
Be your own judge. This is a very clear case. You mischaracterized the condition of a yoyo you sold someone and now are refusing to make it right.