As a junior developer, I mean without great experience, what would you do for a client about an eshop? Start from scratch and handcode an e-shop(simple one)? If yes are there any good tutorials you suggest?
Or buy something ready, lets say VirtueMart templates for joomla or a similar solution for Wordpress?
Even if you described yourself as an expert I still wouldn't recommend that you write an ecommerce application from scratch unless
Displaying products on the front end is relatively trivial. The rest of the iceberg e.g. security, user management, the back end (admin interface) of your website, tax calculations, shipping, cart functionality, templates, payment gateways etc. would take considerably longer.
Unfortunately, even if you deliver you would still be behind the competition when your client says: "I want feature x that I've seen on website y" and you have to quote for bespoke development instead of installing or enabling a module on a ready-made application. Unless they're prepared to spend a lot of time and money they will soon look for an alternative.
If you're already using Joomla, I'd recommend installing the Joomla + VirtueMart bundle here: http://virtuemart.net/downloads
If you're using WordPress take a look at ShopperPress or Get Shopped.
There are many more out there (Magento, osCommerce, Zen etc.)!
The most important thing to do is to match your client's requirements with the features of the application you feel comfortable using.