“RA” is retail arbitrage, a popular way for small sellers to make money online by purchasing discounted new goods at a retailer and reselling them on online marketplace with a markup.
Example: Buy something new on sale at Home Depot, resell it on Amazon at MSRP. This used to be a reliable way for sellers to make a profit, with some sellers growing into 7-figure businesses by accessing deeply discounted remainders or wholesale pricing, but in recent years Amazon has cracked down on RA particularly for larger brands and manufacturers that enforce IP using Amazon Brand Registry.
The small sellers flipping a Macbook they bought at Best Buy … those days are over. This video explains why many Amazon retail arbitrage sellers are getting burned when they try to use receipts. I see it every day on the Amazon Seller Central forums, yet new sellers keep trying.