"Buy something and sell it for more than you paid for it." - Father Guido Sarducci.
I would reckon profits as dollars per lot sold, not dollars per item. Figure that it takes about 1 hour to post and process one lot. Set a minimum thresh hold of profit that you want to make on a lot, and post things that you can reasonably expect to sell for that much.
If you want to see what something sells for, do a completed item search on a similar item.
If you don't have enough of your own stuff to sell, you can always sell someone else's stuff on consignment. However, I wouldn't sell anything on consignment until I had some experience selling my own stuff.
Before selling, I would buy some stuff on eBay so you understand what the process is like to the buyer.
Be honest about your items. Be descriptive. If there's a flaw, mention the flaw. Don't be judgmental. Just be descriptive and let the bidders decide if the item is worth it.
Good luck!
Thanks, VSG!