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