Can users choose different currency for price of their ads?

Yes.

In component’s options you can type what currencies will be available for users to choose while adding their ads.

Please note: this is just a currency sign, no currency conversions will be applied.

Settings in options

In back-end open Components -> DJ-Classifieds -> Options -> Global tab -> Prices settings section 

and in -> Price unit list type currencies seperated by semicolon (;) and Save settings.

Front-end

Now when user submits the advert on front he'll be able too choose the currency.