I chose to buy my alembic strings direct from Alembic after one of the sellers I was trying to buy from was not very communicative when I was asking questions. So I decided to play safe, cost a fair bit but at least I know they are genuine.
I too have been victim of fake strings. I decided to buy some elixir coated strings and then I set them aside for use when I needed a re-string which wasn't until about 8 months after. Anyway when I got them out the ball ends were really untidy with sharp bits of string ends showing which I thought was strange for a premier brand of string. I contacted Elixir sending pictures and they told me they were fake and should contact the seller. I did but was unable to get a refund from the seller through ebay because of the time period between buying and raising an issue. Warning to self, whenever buying strings from ebay or on line for spares check them all when they arrive and hope they don't unravel when you use them later.... which is something that also happened to me.