卖 mài

Picture a shop owner slapping a price tag on a pile of goods. That tag is the 十 sitting on top of 卖 (mài). 买 (mǎi) has no tag because you're just taking stuff in. In speech the only difference is the tone: 买 is third tone (mǎi), 卖 is fourth tone (mài). Third tone dips down and comes back up — you're pulling the goods toward you. Fourth tone drops hard and stops — the goods are gone.

多少钱 on its own is the default price question — short, sharp, used everywhere. Adding 卖 makes the selling side explicit: what does this one go for? That version is fine too, just slightly more wordy. Don't point 卖 at the owner: 你卖多少钱 sounds like you're asking what price the owner sells for. When something isn't on offer, the shop says 这个不卖, not 不买.

卖完了 (mài wán le) is what you'll hear when the shelves are empty. 完 adds the idea of 'finished', so 卖完 means sold out, and the 了 simply marks that it already happened. For 'not sold out yet', say 还没卖完. If you want to ask whether more is coming, just say 还有吗? — you don't need 卖 in that question at all.