I mostly make our costumes. This year I have a Captain Hook (came out very nicely) and a witch (so-so). I, too, am a self taught seamstress who is not very good. I’m making my own costume (a carrot) out of paper. Hee hee. Won’t the kids be embarrassed to be seen with me when they get older!
Make them, (or made them, my kids are both too old now), but those storebought ones, UGH, cheapie little crappy looking things.
Not to mention then your kid looks like every other kid out there.
Plus it’s fun for the kid, they have some say in what they want to be, and can help too (the older ones).
Make them. This year Kiddo is a white unicorn; it was fairly difficult but I pulled it off.
Make them…last year was Eeyore! This year is Winnie the Pooh. I’m something of a seamstress, so it’s not hard, but it involves lots of swearing late at night the week before Halloween. I like to procrastinate.
FB
Yay! I got the vampire-kid to change her mind…now she’s going to be a little old lady. Far easier to whip up on the day before Halloween. Now I get to make a little-old-lady-hat with flowers and birds and fruit on it. 
One year my daughter was the Pink Panther, in pink footie pjs, pink face paint, and pink felt ears and a tail. One of the easiest costumes I’ve ever made, AND she enjoyed being Pink Panther for some time after that as well.
I’ve always hated those cheap plasticy costumes at Kmart and Walmart, so I usually ended up spending a fortune making something. It’s about the only time I ever sew anything. My kids have been a pumpkin, an M&M, a pirate (with black satin pants and a cool green vest), Cleopatra (lots of fake jewels glued on), a witch with a glittery black cloak, and so on.
This year my son is the color red - red sweatpants, red hoody, red face paint, red gloves. I like that one - I didn’t have to sew anything!