A serigraph is a silk screen print. Warhol popularised this method. If you buy a screen print, you'd at least want an edition, a numbered and signed print.
I would not buy anything online, not because it might be dodgy but for personal reasons. Why do people buy art? For many reasons, it may be for investment purposes and it may be because you actually like it! If it's the latter (or both reasons), it's a good idea to actually go and see what you are buying. A work of art can speak to you, and that's why most people buy art. Also, speaking to the seller, seeing the piece, the anticipation, the doubt, the fear of buyers remorse etc all makes for wonderful arty foreplay. It's a lot more satisfying, meaningful and fun visiting the seller and seeing a piece of art before buying it.
I can put you in contact with
someone who is a most interesting and reliable fellow (In Zurich). I recently purchased something quite different from him - by a UK based X-Ray artist called
Nick Veasy.
Otherwise you can buy some nice editions from renowned galleries like
Fondation Beyeler in Basel.