I think I have some feed sack fabric but am not sure. I did some research on other sites and on ebay. I noticed on ebay that some people are selling "feed sack fabric" in yardage. From what I understand they were smaller pieces. Was feed sack fabric available at fabric stores by the yard?

If your fabric was a feed sack, it will have traces of the stitching holes along the sides. Look carefully. Why would anyone buy feedsack fabric when you got it free with the flour? Just curious.

I have never seen by yardage

bigger pieces maybe...and its possible the person who owned it cut bad parts off... so that may not help me if its not there

ok, so those people on ebay selling 5 yards of feed sack dont really have feed sack

There have been reproduction feed sack fabric over the years. Authentic feed sacks are about 36"x45" and is printed on a slightly heavier weight fabric. Stitch holes in the side are also a good clue.
You an find some on the net that have been cut into pieces at an exhorbient cost. Even full size are selling from $15 up per sack.

a few years ago (within the past 5) a couple manufacturers--the ones that specialize in 30's fabrics, put out a line of feedsack fabrics---they are on the bolt- a nice base white cotton with darling feed sack prints from back in the day---
so in theory- the ebay sellers could in fact have feedsack yardage

Back then, they used feedsacks as a marketing tool for fabric sellers. They kept track of the prints that were selling well, and which ones didn't. The prints that sold well were reproduced into fabric yardage. Pure genius, right?
(The average woman needed 3 sacks of the same print to make a dress for herself. When the feed companies started packaging their products in these type of bags, the women were seen more and more in town shopping. I guess they were afraid their husbands would pick up an ugly bag. lol)
As redmadder said, look for the stitching holes in it to determine if it is really from a feedsack or not. The print might be exactly like one found on a feedsack, but it might not be something that once held flour, corn, or salt, etc. (That would not be in a yardage.)

I was told by a long-time feed sack collector that the
fabric was sold by the yard as well as made into bags.

great, I think I have some that is by the yard. but I could be wrong