You have to check the copyright and/or pattern sellers policy. Below is a list of heaps of pattern makers and if they allow you to make and sell or if they have restrictions.
Audrey and Maude (limited- check with them first)
Cindy Taylor Oates
Lil Peas and Carrots (etsy)
Lori Lynn Designs (etsy)
Make It perfect (up to 3 per pattern sold; after that, $5 tags and labels required for each garment from MIP)
Nicole Mallalieu (interpret patterns with own design ideas)
Pink Fig (limit to 5 and acknowledge "Pink Fig" Purchase licence for larger scale production for sale in shops on on line)
RomeoandMae (etsy) Credit for pattern and link to store appreciated
Sew Mama Sew
Tie Dye Diva (etsy)
Bit of Whimsy Dolls (etsy) Only up to 5 in TOTAL at any one time online; unlimited for craft fairs-acknowledge on hang tags.
Enid Gilchrist (acknowledge Mrs Enid Gilchrist)
Funky Friends Factory- requests web address on swing tag- www.funkyfriendsfactory.com
Melly and Me
Napkitten (etsy) acknowledge in description
Precious Patterns (etsy) Only up to 5 in TOTAL at one time for on-line sales; unlimited for craft fairs.
Rosie Quinlan designs
Samantha Caffee (credit designer as "The Handmade Dress" on sale item)
Wired Up Designs
YES- see limitations
cinderella zwergen mode- email for permission first
Little Bird- enquire on individual basis. Hang tags stating www.littlebirdpatterns.com and acknowledging designer required at market. Not for sale on websites such as etsy or in retail stores.
YES-purchase of license required
Colette patterns- email Sarai (approximately $2 US per garment)
Favourite Things (see post 88 for more info)
Indygo Junction (resale request form required and hang tags- http://www.indygojunctioninc.com/sto...hp?info_id=119
Oliver and S (license and label required for each garment- www.oliverands.com )
Sew Liberated (yearly or one-off license- www.sewliberated.com )
Tueddelbude (see post 97 for more info)
Bananafana (consent required)
Montessori by Hand
One Red Robin
Creations by Michie
Kari Me Away (have emailed)
Onion (have emailed numerous times with no reply)
If you do a search on the subject, several bloggers have written very informative posts on the subject. I can't speak for the US copyright laws, but here in Australia you can't just 'change it by 10%/15% ect" to make it yours.
Having said that, some items of clothing like t-shirts for example are so basic, and so many patterns out there, it would be difficult to prove if you have used a specific pattern
Hope this helps