Yes, bubble & poly mailers are great but try to find them at under fifteen cents per mailer when bought in bulk.
This is a Very Competitive Category & your overhead must be very low to survive.
Best Advice, buy in bulk & try to keep your item cost south of three dollars.
Check the Completed Listings in the category you wish to sell in & price accordingly.
In my category, the days of pre-owned shirts, unless they are very unique, garnering Big Bucks are over. These shirts have been in a downward spiral the last three years.
Shipping rates must be competitive with USPS Rates. Be prepared to lower shipping costs two to three dollars.
Offset the cost by adding the shipping loss to your item price if possible.
Don't be discouraged at the beginning if things don't work out the way you envisioned, It does take a while to get established.
Best of Luck.