Providing Ecommerce - Is it hard?

Hey guys, now i've been around a fair while and done various sites etc, but never delved into ecommerce.

Now im done with education and well on the way to setting my own venture up, I realised, that building ecommerce sites for customers is going to be needed (its a web design company).

Therefore I'll ask a few questions to the knowledgable lot here!

Is ecommerce difficult to set up?

Are there effective pre-built packages available - do they cost.

How easy would it be to link to a stock list, for example a list or products/prices already drawn up.

Are they customisable very easily?

Also if you've had any previous experiences setting up/building/implementing an ecommerce site, i'd be interested to know your thoughts.

I look forward to reading some responses!