I have been selling for years. I can tell unless the object is heavy, then Priority is the most preferred method of mailing for both the seller and buyer. For the seller, Priority boxes are free and you can even get them sent to you free from the post office. Plus it gets to the buyer quicker and if you purchase your postage through the USPS.com site or Paypal, you get free Delivery Confirmation. That way no one can say, Hey I didn't get this package! For the buyer, the same thing...it gets there is a few days.
For heavy items I use UPS. I purchase the postage online and ask for a pickup.

When you list your items it will give you a section to add in the weight of the item as well as handling charges you which to include. Since I use the post office boxes, I don't add in handling and this seems to gain more customers. They like that I am not 'overcharging' them. I figure I made my handling when I sold my unwanted item already.

As far as stuff, sometimes when I am out at the thrift stores and yard sales I will occasionally see things that are cheap that I know will resale well and purchase them. I do not however devote time in doing that, I have too many kids! LOL For now I just sell unwanted items me, my family or friends don't need that have some value.
This year is a good year for us on Ebay. We will claim our sales on our taxes cause we are making quite a bit. Usually since I am not an avid seller I would make less than $1000 a year. This year has been very good for us. Mostly we have sold the years of various car parts our family had amassed.

God Bless,
Michele