Yes and no.
We do not allow for Retweets to be the actual transaction tweet, however when someone Retweets your listing, others can purchase directly from that RT. In order to ensure this works properly, make sure people use the native Retweet function on Twitter, rather than saying RT and copying the text.
Facebook shares work a little differently. If a user shares your sale, people can still transact with it if they click the photo/listing and comment. If people attempt to comment directly on the post on your wall/newsfeed the purchase will not work.