The characters for 'buy' and 'sell', seem to have a subtle difference in the character.
- 買/买 (buy)
- 賣/卖 (sell)
I want to know, what's the significance of the radial on top of the sell? How does adding that change 'buy' into 'sell'?
The original form of the top part of 賣 is 出.
出：拿出 take out
貨：交易 to trade
買 refers to the commodity trading activities, both "to buy and to sell", and later refers specifically to purchase transactions.
Your post has typo,
Should be like:
买/買(buy) 卖/ 賣 (sell)
The former one is the simplified Chinese, the later one is traditional Chinese, the 1st pair simply means "buy" or "purchase", the 2nd pair means "sell"