I do not know a rule or guidline. If eBay selling is something that you really feel that you are able to do, by all means give it a shot. However, I would suggest that if your volume is less that 20 items a month or less that $1,000 per month, I would think that it is better to consign your items and do something else with your time. These are just my initial thoughts on the threshold amounts that for most people would work. Consignments are specialised, with Spoo specializing in the more high value items.
As you can see by this thread, one can spend a lot of time listing, selling, collecting, shipping, and dealing with the after sales side of the transactions. Ebay makes it more difficult now for the small time seller, i.e., holding your money for two weeks or more and limiting the items that you can list.
Hope that this helps and good luck.
That's really helpful, actually. Thanks!