So very true! Many times offers come up for coins outside my main interest at very good prices which I might buy to sell on, or expand my collecting interest.
Just another avenue, along with dealers' sites, personal contacts, traditional auction houses, coin fairs, internet only auction sites (check Catawiki once a week), general auctions via The Saleroom et al, and so on
It's totally buyer beware if buying over Facebook, unless paying by Paypal G&S or high value by credit card (rarely offered)