The words for buy and sell have the same pronunciation in Chinese language. Even the characters look similar.
买 (mǎi): buy
卖 (mài): sell
Thank god the tones are different. So make sure you get the tones right.
If you want to 买 (mǎi) then why not learn some of the words below?
买东西 (mǎi dōngxi): to go shopping
购物中心 (gòuwù zhōngxīn): shopping center
商店 (shāngdiàn): shop/store
Whenever I went shopping in China, one phrase I used a lot was
太贵了 (tài guì le): too expensive
贵 (guì): expensive
Make sure you use it a lot too and always ask for a discount
打折 (dǎzhé): to give a discount
便宜一点儿 (piányi yīdiǎn er): a bit cheaper
便宜 (piányi): cheap
免费 (miǎnfèi): free