stillclock I don't know if you can ever guarantee if you will use or like everything you get in a fabric swap. You also cannot guarantee quality of fabric. Some fabric will be LQS some big box stores and some Wally World. It is a mystery.
If these are your concerns, you may want to consider buying scrap bags from some of the fabric shops. I have purchased several scrap packs from online shops such as
JAQS Fabrics,
Hawthorne Fabrics and
Fabric.com. On another scrap related thread someone mentioned purchasing scrap packs from
eQuilter.com. While this is not trading scraps with other quilters, it is buying a variety of fabric scraps of LQS fabric for you to play with.
Also consider visiting yard/estate sales. Hopefully you will find fantastic fabric stashes from which to choose new fabrics to play with. The last moving sale I visited for fabrics, I came away with so much fabric I am still trying to incorporate it into my stash. I had to start a whole new box to house it all. I even got a half yard of Vera Bradley fabric in that sale.
There are several options for you to get new fabric scraps to play with. I hope you find something that will work for you.