I use the plastic grocery bags for:
Garbage bags in the bathrooms or and small garbage can
To pound out chicken breasts for scallopini
Lunch bags
Poo bags for the dog
Wrap and tie a paint roller tray for easy cleanup. The handles wrap right around the little legs.
To give away excess tomatoes (lol)
I keep them in a drawer in my kitchen for easy access.
That's exactly what I use mine for! (Except the dog thing, I don't have a dog).
There's no way of knowing what you have to price the item in order to make a profit. Generally the rule is twice the cost of the items needed to make one bag, plus your time. Be realistic in your pricing and if the bag is a good quality and the design of the fabric is appealing I would pay up to $5.00. But that's me. Try asking different prices for bags that are different in the original cost. If they're well made they will last a very long time. You might want to sell them on a wholesale basis to specialty shops as well. I once owned a cat themed shop and we could have blown cat designed bags out the door. Same with my year round Christmas shop. I would love using bags with a Chirstmas pattern. Use your imagination and I hope you follow through.
I'm guessing the durable fabrics would cost around $4 or $5 a yard, and it will probably take close to a yard to make the bags, so it may not work out so well. I like the specialty shop idea though. People tend to be more willing to spend at those.
I've been using heavy-duty woven nylon-coated bags for almost 25 years. Bought them back in Maryland from our Giant Food grocery chain. Liked them so much, I made three more out of rip-stop nylon. All my bags are washable and none of them show any signs of wear. The rip-stop nylon ones fold up/ball up so small they can almost fit in my pocket.
As for the plastic ones the stores use, I save a few for the purposes already mentioned. The rest, plus any produce bags, bakery bags, frozen vegetable bags, etc. all go into the recycling bin.
Where did you find the rip-stop nylon? That sounds handy? I know I will have a hard time wanting to haul re-usable bags into the store with a car seat and a toddler. The thinner (but stronger) the better so I can just stuff them in a pocket or diaper bag.