Some people are nervous about making purchases online

because they are afraid that the product or service they receive will not be of the same

quality as advertized on the website. So we can say that they are worried about

the possibility of buying "a cat in a sack"

which is exactly the expression we use in Hebrew.

Another option is to say  "a pig in a poke"

(a "poke" is an old English word for bag),

but hopefully you will not be "ripped off"

(tricked into paying more than the item is worth).

Happy and safe online purchasing !