Personally, I think an online business is just too risky. There are so many things to consider:
Are you going to take credit cards? If so, you will need to figure in the fees you will be charged for transactions.
How much does your bank charge for a business account? How much will the bank charge for nsf checks and return charges?
Don't forget shipping charges to you as well as to the customer. Plus you will have shipping supplies to purchase. That can run a few dollars too.
How will you handle a customer that states they have not received their order? How will you handle returns?
Then, if you do give a few extra inches on each cut of fabric, it won't be long before you have given away a couple yards from each bolt of fabric.
And I'm sure there are many more things that need to be considered as well.
You really need to put a business plan together before doing something like this. Talk with some shop owners and ask them the pitfalls of the business. Ask how long it was before they had an actual profit from the business.
I can understand your discouragement with poorly cut fabric. When the fabric is being cut, if you see the fabric is uneven mention it to the person cutting the fabric. Fabric distorts being wrapped on the bolt. Most of the time I just order 1/4yd more than I need. I feel this will allow for shrink factor and squaring the fabric.