A few starting places you may want to consider Paul is eBay and Audiogon. Ebay may have some other sellers of the Infinity Alpha speaker line and you can somewhat gauge the going price if you're able to find some of the Apha 50s for sale.
Audiogon has lots of classifieds listings, currently doesn't look like there are any Alphas being sold, though. AudiogoN also goes so far as to provide used pricing for most A/V equipment using their Blue Book feature. This is ideal, but also comes w/a price of $20/mo., $50/yr, so that likely won't be terribly cost-effective for you unless you're selling more equipment, but it can obviously give you the most realistic value of the Alphas once you've factored in their age, use and condition. You may also want to strongly consider selling locally if possible, as the shipping costs of these guys will likely eat into your profit margin and may not be worthwhile to ship.
Good luck w/your sale and let me know if I can help any further...

-TD