It is illegal to charge others for material copyrighted by another organisation. Therefore, you are legally required to advertise and self yourself as someone who sets up a template, and not as a web designer. You must ensure that all your clients know what it is you actually do, and that all the materials on their website are otherwise copyright-free (no Google Images).
Another legal requirement is to pay tax. You should check your local government to see if you would be elligible for income tax and pay it accordingly. You may also need to apply to become a company in order to trade.
Of course, you could disregard the law, but make sure you understand what could happen if you get 'caught'. I started web design at a young age, designing casually for friends. However, I could write basic HTML and CSS, and I used Wordpress. £200 ($350-ish) is a good amount to charge at this level for a pretty simple web design, but seeing as you're not actually desiging anything yourself, it would be immoral to charge this. My first rule in business is never to rip anyone off. I have friends who charge £20,000 for a web design - I will charge no more than £1000, as a full web design from scratch, because I just don't think websites are worth that much.
Just stick to £30/$50 and see how it goes. Don't over-charge; it's immoral and clients will only be disappointed when they end up with a Wix-branded website and a template that they could have set up in the first place. This is an industry that can work, but rarely does, and Wix is not made for resale - it's a very basic and unattractive product and your end-user will not be able to justify having spent a large sum of money on it.
Also, the idea that designers can't be good at coding and vice versa is simply not true. The best web designers will be able to come up with a great website on their own, but a better website with others. This game is way too competitive for you to be good at 'one thing'; you can only get ahead if you have any eye for design and can write at least intermediate level PHP (static websites are very undesirable nowadays; you want as much dynamic content as you can get your hands on).