Seriously, the one thing they ABSOLUTELY need is A GOOD STRATEGY!
Which particular shopping cart or payment system, or any technical detail is INSIGNIFICANT when compared to having a good strategy.
I say this not as an expert, but as someone who has been muddling their way through ecommerce for 12 years and still finds it a challenge to pay the rent.
After all this time, I am finally, FINALLY learning what it is going to take to succeed at ecommerce, much thanks to the people on this forum.
so the first thing they need to do is figure out:
1) Do people want the product? (Maybe try selling on ebay or amzon stores or something like that first to make sure there is a market for the product).
2) Decide if they will be able to win out over the competition? Can they say why people should buy from them instead of every other guy and their dog who are selling the same thing online? (And no, saying "people should buy from us because we need the money" is NOT valid reason.) They need to know EVERYTHING about their competition - what their prices are, where they get their customers from, what are their strengths, what are their weaknesses. EVERYTHING.
3) Do they know how they are going to get the word out to potential customers? They need to know how much time and money it is going to take to get customers to their website. Because you can have the greatest product in the world, and if no one knows about it, then you are still hosed.
I am not trying To be overly pessimistic. There are a lot of people who started a lot later than I and have been WILDLY successful. That gives me hope.
But at the same time, there is a vast graveyard of domain names for sale. People registered these domains planning on making a living online, and gave up after a couple of years, or months, or weeks, once they saw that nobody wanted what they were selling, or the competition was selling it for less, or they couldn't get anybody to come to their site.
So strategy is 95% of success online and all the other stuff is only 5%