Myth check
If I ask 多少钱 I must bargain or I am rude.
Asking the price is normal. Bargaining is optional and depends on the stall, not on your textbook.
Verdict
mostly_false
Why
这个多少钱？ is a complete, polite question with a pointed item.
太贵了 and 便宜一点 are usable lines. 不行 plus 好吧 is also a complete exit.
Supermarkets and posted prices are not night-market theater. Use the sentences that fit the room.
Better framing
Learn the price question first. Add bargaining only where locals actually bargain.
Practice tip
Role-play pointing, asking 多少钱, then either 我买这个 or 太贵了 without mixing 买/卖 tones.