I don't (yet) own a sewing machine, but here are ways I'd answer those questions if I did:
-- Yes, I'd be glad to mend it if you'll watch my kids for the afternoon (or whatever favor you seek). Or just do the mending and then feel free to ask a favor down the line. I mean, maybe they already have something you'd like in return, like free legal advice or they make awesome cakes or grow tomatoes.
-- Yes, I can make things like that, but I don't do it as a profession.
-- What kind of help did you need? I'd be glad to try helping with a tough spot, but if you need guidance with the whole item, it would be better for you to sign up for a class somewhere.
-- Sorry, I only do this as a hobby. There's an alterations place on _____ Street. Or Mary down the street also sews, and she takes in sewing to earn extra money.
-- Sure, I can do that that for you. My fee is $_____. Or, I'll swap you for it.
-- I'd love to take a look at your fabric, sure! (And then if you can't use it or don't like it, refer them to eBay or the freecycle section of craigslist.com)
-- Yes, I quilt. Why not try making your own? Then refer them to a class or the local quilting guild. Or, if you're so inclined, start a quilting circle with the two of you and others you know.
-- Yes, I quilt, but it's not like stringing beads for a bracelet. Quilting takes a lot more time and money, so I don't make them as favors.
-- Sure, I'd be glad to do you this favor. But please understand that I don't do this regularly for people. If they ask why not, just say it's because you don't want to, or you'd put out a sign and charge for it.