Check with the main administration of the thrift store. 6 or 7 years ago, I reported some bizarre stuff about our local Salvation Army store (high prices, unusual items that aren't appropriate) to the main administration and the manager was arrested several days later (apparently a lot more going on). Same thing happened again last year - new prices for used junk. I again contacted the main administration and there was a new manager in less than a week. They don't make any money if the merchandise doesn't move, but the high prices may indicate that something is up.
I have no issue with higher prices for NEW stuff (Target donates unsold items periodically), but they want merchandise to MOVE and it isn't going to if you charge near new prices.
Instead of sheets, consider buying low end muslin or making smaller projects until you feel comfortable with your machine. Hancocks had some $1.99 muslin that was 90 inches wide not too long ago. I personally don't bother using 'used' stuff to make quilts. For practice, it's just as easy to get something off the calico shelf at Joann's or use a coupon and get the 90 inch wide fabric. It will last longer than used fabric.