OK, so you're buying and selling things via PayPal or regular checks and money orders. Half the battle is the shipping of products. Most people still go to their local Ship On Site or UPS Store, hand them some goods, and pay out the nose to have them box and ship the item(s). It's seriously expensive! Typically they'll want $25 to box-and-ship some small item you can ship yourself for $8 or less. Instead, you can handle all of the shipping from the comfort of your own home and never have to leave the house! 

Here's how:

• Collect boxes.
• Save boxes from products shipped to you. You can easily break them down flat and store them somewhere.
• Get totally FREE boxes from USPS (US Post Office). They will even deliver them to your door. FOR FREE! You pay nothing at all! That goes for all Priority and Flate Rate boxes, stickers, labels, everything but packing tape.
• Collect packing buffer. I save bubble wrap, and keep a garbage bag full of packing peanuts, then reuse as needed.
• Get packing tape. This you'll have to purchase. But it's inexpensive if purchased at Walmart, BJs, Target, etc.
• Get a scale. I currently borrow my wife's 5lb cooking scale to weigh my smaller boxes. Larger boxes I just take to my local Ship On Site and they happily weigh them for me for free. But most items I personally sell are almost always under 5lbs. You can buy a 5lb scale for $19.95 or less, a 20lb or larger scale may cost up to $50. But if you do a lot of shipping it's a worthwhile expense to not have to leave your house to weigh items.

• Always pay for and print your shipping labels online. You don't have to leave the house, you get easy access to tracking information, and best of all it's the cheapest possible way to pay for shipping. For excample, USPS saves you at least a couple of dollars thanks to discounted prices and free delivery confirmation, which normally costs $0.80 at the counter. No matter what, shipping from home is always seriously cheaper than going to Ship On Site or the UPS Store.
• If you are shipping small or light items (guitar effects pedals) ship them USPS Priority. It's typically the least expensive method to ship these types of items, usually $6-$8 depending on the weight and how far it has to go and whether you opt to pay for insurance.
• If you are shipping small but heavy items look into USPS Flat Rate. You have to use one of their boxes, but as long as it's 70lbs or less you pay the flat rate which I believe is $13 maximum. However, I've yet to find anything that's heavy that'll actually fit into their boxes. They never seem to be the size I need. And then often it's cheaper to ship using a different carrier.
• If you are shipping items that are large and/or heavy (rack effects, amps, guitars) or are travelling more than a few hundreds miles it's almost always cheapest to use FedEx Ground. I shipped a 35lb amp head 1/2 way across the US for a mere $17. Guitars are usually 30 or less depending on how far they have to travel. Another benefit is FedEx provides $100 worth of shipping insurance free, at no cost.
• If you ship USPS Priority or Flate Rate you can handle the shipping labels directly in PayPal. Just go to your account, and click the "Print shipping label" option for the transaction. PayPal will fill in the addresses, you just plug in the box type and weight, click a couple buttons, the fees are taken from your PayPal account, and it prints the paid shipping label on your printer. You just tape it to the box and you're done! Leave small boxes in the maibox, drop them off at any USPS location, or even drop them off at Ship On Site or UPS Store, they happily accept prepaid items.
• If you ship UPS you can still process that directly in PayPal as well.
• If you ship FedEx you'll have to process that directly on the FedEx website as there's no PayPal integration at this time. But you can still enter the tracking info into PayPal. They have a handy link in the transaction details to help you do that.