Great question.. First of all OpenCart is a great platform (personally my choice vs Magento)
Developing a site is easy - you just pay someone a set amount, follow some structural guidelines that work, and you have a website.

The hard part actually comes in when it comes to managing e-commerce process. It's similar to building a restaurant. You can hire a builder to make anything you want. The trick is how to get people to the restaurant that actually come in, come back, and refer other people.

From my experience (as a Director of E-commerce), if you were to partner with someone it should be E-commerce professional who knows how to measure key KPIs (key performance indicators) and effect of the KPIs to the brand and Sales. It's easy to outsource social media but it's hard to know if the social media is done properly. It's easy to outsource PPC, but it's hard to measure what works and what doesn't. It's easy to outsource product upload but it's hard to outsource understanding what product description should say.

Here are some examples

1) Anyone can post funny images on Facebook. Is that considered to be social media marketing? What is the ROI? How many people visit your website? How many people who came from Facebook buy?
2) Uploading 100 products is easy but what is the conversion rate for the product? Are people staying on the site, are product images optimized, are product descriptions written in the way to emphasize the benefits rather than just listing features? Are you driving targeted PPC and SEO traffic to that product and measure e-commerce conversions and page value?

3) E-mail marketing is easy. Understanding how to capture the leads, who to send e-mail, what kind of e-mail to send, and when to send e-mail - requires knowledge. Remember if you send an e-mail to people who are not interested or don't remember who you are, you're risking above 1% spam rate. Most e-mail providers would frown upon 1% and above spam rate and might even cancel your account. Another thing, if the e-mails goes out to people who are not qualified and/or not interested, you're risking a very high bounce rate- this is BAD for UX (user experience) which is ultimately bad for SEO.

So if you were to partner with someone, think about partnering with someone who knows how to put all the pieces together in order to drive quality traffic, generate SALES, and follow up to get more sales. Outsourcing and development is easy, understanding WHAT needs to be outsourced and what to look for - that's the important thing.

Let me know if you have more questions, I would be more than happy to answer them.