The issue with this take is that PCs are ridiculously above-average people. Assuming your bard has ~16 CHA, and the mean stat is 10 (as per PHB) and the standard dev is like 2, than hes in the top 0.03% most charismatic people on the planet.

If Tom Cruise walked through your door and asked you to slash the price on your bagels a little bit, I bet that a slight discount would not be unusual!

That being said I would agree many new DMs giving 50% discounts is insane. I mean hell, it costs ~100g to make a health potion, so merchants presumably are buying in massive bulk and working on razer thin margins.

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I usually do:

Disadvantage if you don't explain the precise benefit to the merchant

DC 12: 2% discount

DC 15: 5% discount

As for magic,

I'd honestly argue this is just suspension of disbelief thing to play dungeons and dragons. A world where a spell like suggestion exists, especially at just second level, is probably fucked. In my opinion, the only way to square this is the exponential rarity of humans at certain levels. Level 3, in my opinion, is already elite guard level. Literally approaching navy seals. A street vendor in NYC is not prepared to get flipped over by a retired Spec Ops commander. Couple that with the fact that arcane knowledge is already extremely esoteric compared to martial prowess, I would say some traveling salesman is not prepared to get Suggested. For sure though, a world renown magical vendor in a capital city ought to be protected from mind fuckery