Second - Online services like MonsterCommerce or Mountain Media have the benefit of letting you get your feet wet without the up front expense of building something.
The added advantage is that you're going to save some additional money (and the pain from smacking yourself in the head) when you realize that the functionality of the custom cart you bought isn't quite what you wanted.
Better to start out easy - especially if you don't have a webjockey on staff to help guide you - and take notes as you go!
That being said, there will come a day when you realize that you're limited by these online services. They may work well for you for a few years or more or just a few weeks. It really depends upon how much you learn and how quickly.
The next logical progression will be to move to an off-the-shelf package and have it modified to your liking. These come in a variety of different flavors. Between now and then, research and read and keep notes on what you do and don't like about the service you go with!
Wow, one of my first posts got pinned in the ecommerce101 thread (woohoo).
Anyway, this link is from ecommerce 101 (lorax's suggested reading) and should provide some help on your question: [webmasterworld.com ]
We had a similar decision to make about 9 months ago and ended up going with Volusion. We felt it was a more scalable solution than Monster, had a better look and feel, and was infinitely better at handling inventory controls. We've been very happy so far. Not perfect, but darn near close to it. We're just about to the point where we'll buy the license for their software and hire our own web guys to do the work. Until now, its just been myself and my partner managing the site with our intermediate HTML, ASP, FLASH, etc...knowledge
[edited by: ItsAllBallBearings at 5:38 am (utc) on Dec. 7, 2006]
I maintain stores with Volusion and 3dCart. We had a Monster store for a number of years and found that both Volusion and 3dCart offer more features and fair better in the SERPS.
Look a few different carts and even try the free demos. Decide on which solution offers the best tools for your business. There isn't a one-size-fits-all cart on the market. You have to really do your homework as it is a major decision.
I have a few monstercommerce stores and have had great success with them in fact I just started another one.. I must admit that since Network Solutions took over I think the phone lines get mixed up between old MonsterCommerce Clients and New that is the only thing I see they need to improve. I like the vast amount of tools you can use to beef up your store. You have tons of design features and backend SEO enhancements most are unaware of.