Thing is, by the core's implied setting, a metropolis like Waterdeep is definitely the exception. It's more like finding that one travelling merchant in the village of Auckney that has enough cash to buy the item off you, but he's only going to pay 1/5 market value for it.
Also, posting an ad like that in a metropolis: you don't think that advertising that you had such valuable commodities would make you a target for theft, B&E, or murder? Seriously, magic items are fairly small, light forms of great wealth, they'd be prime targets for the criminal underground, especially when magic is rare. That's why the people that devote their lives to merchanthood are the ones who trade the items - they have the connections and contacts to (relatively) safely resell the items without advertising (and getting stabbed and robbed).