I have made my living selling on eBay for the last three years. I started selling when my firstborn was about 6 mos old as a way to get out of a traditional job and stay at home. Now I resell for large companies that are liquidating overstock and obsolete business and industrial equipment. I have found it is a good way to make a living, but like a traditional job, it is still really hard work. I have a 4-month old at home now too! It can be quite time consuming, but is not hard to do at all once you get the hang of it.
You do not have to have a tax ID, although you should report your income each year when you do your taxes. eBay fees are high at times, it all depends on what you are selling, how much you are selling it for, etc.
There is a strategy to selling to maximize your earning potential. I have a lot of experience with kids' clothes and I have found that designer clothes and shoes sold in lots sorted by size sell the best and make the most money. Don't expect to get back what you have in the items, even for items that are new with tags, unless they are very high demand items. eBay will typically bring in 50% of retail value, so you will want to price your items realistically. Say you have a lot of 4 pair of name brand shoes, Children's Place, Gap, etc. You have maybe $60 in all the shoes. You'll probably be lucky to get around $20. More expensive items like Stride Rite shoes you can sell individually, but should still plan on getting back about half of what you paid for them, assuming they are in good shape.
You will want to mark up your shipping fees to make a small profit, either by increasing your handling fee - realistically, no more than $5 for packaging and transport to the post office, FedEx or wherever. USPS Priority mail is popular and affordable for small, light items, while FedEx Ground is the most affordable for larger, heaver items. UPS and DHL are the go-to guys for HUGE, HEAVY items. For International shipping, use USPS International Priority - they are the easiest and cost the least on duties and taxes on the receiving end. To quote your shipping cost, go to USPS.com or FedEx.com and click the links to 'calculate shipping', add your weight and package dimensions in there and it will spit out an estimated shipping cost. I usually take that cost, add $5 and put that as my ship fee. I have had no complaints and have a 100% feedback rating.
Your overhead will be:
1) cost for your time
2) eBay fees (they charge listing fees on the front end and final value fees - a small percentage of your sale price on the back end)
3) PayPal fees (2-3% of your sale price - this is kind of like a credit card transaction fee)
4) packaging fees for supplies like tape, bubble wrap, etc. USPS Priority Mail boxes and envelopes are FREE and they will deliver them to your house if you order them from USPS.com
5) GAS to drive to the post office
Make sure you have a good digital camera, a scale - a digital bathroom one works, but make sure it will read items under 5 pounds, as most won't. And ALWAYS describe your items completely. Has it been worn/washed/dry cleaned? Any stains or defects? Does it come from a smoke/pet free home?
Make sure that you have the time to devote to maintaining your auctions. A standard auction can go for 1 day, 3 days, 5 or 7 days. Most activity on auctions that do not have Buy it Now options happens in the last 2 days. You want to list your items at night so that the auctions will end at night - when Moms are at home on their computers bidding it away after the kids go to bed. I sell business equipment, so I start and end my auctions during the week during business hours to reach my core market. Make sure you check in frequently to answer any questions potential bidders may have about your items.
It is also good business to offer a shipping discount if someone buys multiple items from you. You can waive your handling fee or discount it in some way. I do not suggest shipping multiple items in the same package though because I have had buyers that tried to say they didn't receive something when I KNOW I put it in the box just so they could get a refund. Which brings me to...
Crazy eBay people. They are there, you can't avoid them. It doesn't happen often, but when you get a nutcase, it is always a MAJOR headache. The new eBay rules do not allow sellers to leave negative feedback comments for buyers that flake out on paying or who cause headaches, so it is harder for sellers to protect themselves against crazy eBayers.
Please feel free to email me if you have any questions about how to get started, etc. eBay has great tutorials on how to start selling. I use their Turbo Lister application to list multiple items at once. It is free, and not the most convenient tool of its kind out there, but it works for me.
I started like you, selling stuff I had around the house like old printers and baby gear. Then I went to work for an eBay store and I sold things on consignment for people. I have sold everything from Barbie dolls to Harley Davidson motorcycles and everything in between. Now I operate from home consigning things for businesses and have found it a great part time job to enable me to stay at home with my second baby. Now I am a PowerSeller on eBay and I am about to expand my business. Good Luck and please don't hesitate to let me know if I can offer any help! eBay is complex, but not as bad as you might think.