For the buyer, they are able to buy the 400 $UNI at a slightly discounted rate and make $10 profit per token, giving them a net profit of $3,250 USDC (gross profit 4,000 less the 750 they paid upfront). Given the Buyer put up $750 in capital and profited $3,250, that's a 433% yield implied 1,757% APY.