I have recently become involved with a brand new web app that aims at simplifying flash template websites for artist portfolios: http://sharemyartwork.com/
I hope you find this helpful - let me know what you think about it.
I think you should be at the very least on Facebook. But that's seriously the very least...
These days a solid web presence is not only a good idea but almost a requirement. Like a said before a face/space page is a start but I think the most important thing you need is your own website and an email that is directly connected to that site. I personally find it to be a huge disconnect when I look at someones contact info and I see there website but a gmail account as their primary email.
Other great resources includes (some of them are obvious)
As far as sales go... I find Ebay to be a little too broad of a website. Try using sites that focus on the arts like Brooklyn based supermarkethq.com or etsy.com. Supermarket is invite only, but it's easy enough to submit your work for approval and once your in, your in. Etsy is open to all.
... and of course get you work up on to this site...