I tend to buy the following:
fat quarters ( I like to blend lots of them when making blocks or scrappy quilts)
Half Yard pieces (fabrics I want more than a fat quarter of but can't afford lots of)
1 yd pieces (fabrics that are a great price but a novelty print)
2-5 yd pieces (solids, linen blends, shot cottons, etc.) Anything that could be used for sashing, setting triangles background etc.
10 yd or more bolts muslin, both bleached and unbleached.
I also buy vintage bed linens (flat sheets) from thrift stores to back my quilts with. Lots of fun picking the perfect one for a quilt and very economical :)
I also love to pick up charm packs and jelly rolls and kits on sale for those times when I want a quick project for a gift or charity.