Sonia, I think Graham might have been in a hurry when he answered. The sentence "The tickets can be purchased over there" is correct, but the passive constructive does *not* use the infinite when you are using one of the modal verbs (can, will, would, could, should, etc.). Modal verbs NEVER use the infinitive. 

The proper construction for passive with any modal verb is:

Subject + modal + "be" + past participle. 

The tickets + can + be + purchased. 

Examples: 

The window + should + be + shut.

The cat + can + be + fed.

Clothing + must + be + washed.