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Facebook could be tracking your online Plan B or HIV test purchases
(www.popsci.com)
This is a most excellent place for technology news and articles.
Possibly, I don't use alternatives much (primarily because I don't sell much in general). I prefer Facebook because so many people are there and it's easier to facilitate cash payments via meetups. But there could be better ones, I'm not all that familiar with Swappa or OfferUp.