Identify the unfair aspect with respect to the sentence extracted from an online Terms of Service:
```
you are only allowed to purchase virtual items from us or our authorised partners through the service , and not in any other way . 

```
. OPTIONS: ['Contract by using' 'Choice of law' 'Content removal'
 'Limitation of liability' 'Jurisdiction' 'Unilateral termination'
 'Unilateral change' 'None' 'Arbitration'].