You can buy 650 sqft of banner for under $100 on E-bay. Even cheaper if buying multiples... that's 15 cents a sqft. These places use cheap bulk ink.
They sacrifice quality for quantity... You can't compete. Let your customers buy from them, then when they get a shitty looking banner that's blue instead of grey, they'll come back to you.
I once paid something like $10 for a 5 FT banner, to which I sent a pantone chart to them. I don't think they had one color right, and some were nowheres near. Cheap material that'll break in a month, cheap ink that will fade, and no quality control on colors. If you cut all them corners, you too can start selling at 50 cents a Sqft! Of course you can't just close shop and start a new E-bay name when all your customers get pissed off at you. So they do have an Advantage on that...