Even if you do not want to buy anything, sellers will try to sell you a souvenir. You just have to say "La Shukran" (=No thanks). Once you have pronounced this sentence a hundred times, the seller may be convinced that you do not want to buy anything.
Why do sellers insist so much? The answer is simple, because even if you want to buy a souvenir you have to say "La Shukran". And you have to keep saying "No thanks" till the seller gives you the price you are willing to pay. Weird Game isn't it?