Is the meaning of 卖 the original meaning in 卖力，卖萌，卖国，卖傻，卖弄? 卖 means to sell. But in the above five phrases, it is very difficult to get the point. If 国 can be sold to some extent，as a piece of land are illegally sold by the authorities to foreigners for personal gains, the other fours look ridiculous, as nobody would sell his or others' stupidity or foolishness?
「貝」(shell) in ancient China was used as a currency. The original meaning of 「買」was 'take money to the market to trade for goods (to buy)'  The top part of 「買」was originally representing 'goods displayed in the market' ; the character 「賣」 added a hand behind the goods to represent 'handing out goods at the market to trade for money (to sell)'

「買賣」means 'to trade' (include the buyer and seller). 

The character 「賣」was originated in an 'market activity' namely 'to display and sell' therefore, it can be used as a verb for 'to display; to show; to promote'  


「賣力」 means 'show off one's hard labor to impress' = 'work very hard (to impress)'
「賣萌」means 'show off one's cuteness' 
「賣傻」 (display dumbness) means 'play dumb'
「賣」in  「賣國」short for 「出賣」which means "to betray" (to betray someone like you are selling him like goods)  
「弄」in 「賣弄」means 'to handle', 「賣弄」(display and handle) means 'to show something off'