Tandems are difficult to sell for a couple of reasons. Market size is very small, since the tandem style is not all that sought after. Secondly, unless selling locally and offering local pick-up opportunity, then the cost to ship the bicycle will be very high, once again limiting the number of potential buyers.
The suggesteed price of $350.00 seems low to me, considering what the bicycle is actually worth, but my guess is that the quote is pretty darn close to what you can expect, unless you run across an uninformed buyer or someone who really wants exactly what you have.
That said, decide what you think you want, for the bicycle, and list the bike at double that number, leaving plenty of room to negotiate. If the first person, to walk through the door, purchases the bicycle for asking price, then you will know that the price was too low, but at least you will have received two times what you thought the bike was worth to you. If you can't find a buyer at that price, you can always come down, but keep this in mind...
In many parts of North America, this is the best time of year to sell a bicycle. Right now, there are lots of people with Spring Fever, all waiting to spring into action, and perhaps buy a bicycle in the process. Act now and plan your "bike for sale" campaign to last about six weeks. If no sale by then, you will have to lower your price a lot, or be prepared to be patient and play the "waiting for the right buyer" game.
"Waiting for the right buyer" game is the one that will help you get the most value for the bicycle, in my opinion.
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"98% of the bikes I buy are projects".