your first and biggest mistake:
you are trading XRP with ETH but you are looking at prices in USD! that doesn't even make sense. you should check the price in ETH not in USD. if it is different then move ahead.
I assume you are using coinmarketcap.com to see these prices because I can not think of any other way you could see XRP/ETH price in USD. that is a big mistake because they report the last price not current price and it is update with 5 minute delay.

second mistake:
you should check the current live price. more accurately the highest buy price in where you are buying and compare it with the highest sell price in the place you are selling. then if there was an actual difference move ahead with sell.

third mistake:
you forgot about fees!
you are paying 2 trading fees to buy and sell (each around $1.8 ) these coins and 1 withdrawal fee to transfer the coin between exchanges.

final mistake:
you forgot about the time it takes.
it is not instant to move any coin around and the market is sometimes too volatile and it keeps changing every second.
all these mistakes are the reason why Arbitrage trading is not profitable and not that many are doing it.