butterfly chair covers  

Judging from the photo, it...
Judging from the photo, it seems ok.
The denier isn't stated...but I guess it's a low, smoother weave versus a higher more durable weave. Seems an economical way to go.
Seller does have a money back return policy. 100% feedback with almost 3000 satisfied customers.

I have a bunch of vintage...
I have a bunch of vintage butterfly chair frames and four of the outdoor covers from Circa50. You do need to get the right size for your frame! My frames are all different sizes and right now I have two out with two random covers on them; one is too big for the frame it's on and it's not very comfortable. It's hard to get up out of the chair!
Circa50 sizes theirs by the distance between the points of the frame. It looks like that ebay seller is sizing his by the width of the cover when laid flat. It looks a little bit big on the frame in his photo.
I would just try to clarify the sizing before buying, otherwise it looks ok. Maybe just get one and see how it looks, then buy more if you're satisfied with the quality and the sizing.

Right size
Spanky is right
It's true that size issue.
As one of the most copied and with more verisons of knock offs furnitures in history of contemporary design, It can be fund lots of "versions" each one with their own measures "reinterpretations".
If I have to choose I'd like to try the cover with my frame.
In the link they show the size of their one.
You can contact them to ask...
http://www.toledoenterprises.com/custom.asp?id=300197&page=2

It says "Indoor Use Only" on...
It says "Indoor Use Only" on that website. I guess that makes my decision....

Outdoor
Perhaps the "Outdoor butterfly chair cover"
http://www.toledoenterprises.com/custom.asp?id=300197&page=1&showsite=true

looks great but horrid to sit in
Is there a less comfortable outdoor chair than the Butterfly chair?

Stephen
A butterfly chair with a cover that is even a little too big is very uncomfortable. But when it fits, it's a whole other story. I love sitting in mine.

I'm with spanky on that
I have repro foldable B-fly chairs from Crate and Barrel. I've had them for years and years, they were my first toe-dip into modernism. I bought the covers from C&B as well (canvas duck) and found that I had to keep washing them and drying them to make them comfy. As soon as they stretched out I began to feel like a geriatric patient trying to get up out of them. I bought some outdoor covers made out of some kind of mesh fabrics, nylon or some other poly and these are much more comfy to sit in. I'm thinking of having leather covers made once I get to NM. Those will be able to stay outsie most of the time out there.

Olive - where did you get...
Olive - where did you get your outdoor covers?

outdoor covers
These are Circa50's outdoor covers. I wish they had red and orange but oh well, they're nicely made and they dry quickly!
You can see in the photo how the cover doesn't dip much in front. This is the difference between uncomfortable and comfortable.
I made some covers once out of heavy duty denim and the fabric stretched a lot even though it seemed quite stable. I had to throw them out.
http://www.circa50.com/butterflychairs.html

Bizarrely, I got them at Urban Outfitters
And equally bizarre, they were quite well made and have stood up really well over time.

Olive
Do you have the folding butterfly chairs or the non-folding?
I ask because a lot of places sell covers for the folding types, which are smaller than those for the original non-folding chairs.
That said, two of my vintage chairs are only 26" across at the front and I think the size B from Circa50 is a bit big on them.

that's it.
Urban outfitters. I have the same ones bought in '87, the year i moved to NY.
My first chairs, probably bought the same day as the futon.
(that was minimal living!)
I've only had two sets of covers and need to change again. I've not tried the
exterior mesh ones but they should hold up better in the sun. My canvas ones
have faded and hold water.
I do like the larger 'fixed frames'. But the folding ones store nicely off season
and folded half way can tuck under the overhand of the deck.
I don't think it should be the only chair choice on the deck but it is good for
a book or magazine. Or a nap. At parties they are always occupied and kids
love them. It's a commitment. The half-hour chair. Once you set yourself up
it is a nice ride.
Has Urban Outfitters really been around that long?

Since the '70's I think for U.O.
I have folders, I like them for the same reason Rockland does. Easy storage. One thing that is for certain, the size cover has to be correct. The photo spanky posted just above is a perfect indicator. The frony of the seat, which would be just under your knees is nearly a straight hoizontal line. If that line dips down the cover is too big and getting out of the chair requires a winch even for the most athletic of individuals. One of the reasons I have come to dislike the canvas duck covers is that you can start out straight and quickly go to saggy as the fabric stretches. Much like the jeans that are tight from the dryer and loose after a day's wear, twill fabrics are like that.