Yes it is arbitrage. I would think it is usually the other way. eBay seems to get higher prices most of the time if it is a well know brand. Rotel, B&W, NAD, MSB, Adcom, Nordost and high end Yamaha, Marantz, Onkyo, etc.
I've actually had dealers buy my used gear; and then it shows up on their website's used/demo page at more. The dealer usually offers a warranty on used/demo equipment, so they can get more $$, especially from walk-in customers. One in particular was a vintage 80s Cambridge Audio amp, back when they were still made in England. The dealer was a Cambridge Audio dealer. I think he was interested in comparing it with the current stuff. He sold it later. The rest were mostly Tuners.