ChristianeD,
in my experience, they are not junky,they are fine quality fabrics, mostly discontinued lines (who cares)of famous makers but there are only a limited number of yards left. sometimes they will ask if you want them to ship what there is if they don't have the full yardage of a particular fabric. i always say yes. you can so no. sometimes by the time you order, there isn't any left. there is never a full line of any designer's collection. some designs come in more than one color, which can be very interesting. in other words, it's hit or miss. if there isn't a lot that i like, i pass it by. i want to save on the shipping.
some sites will not charge shipping if you spend over a certain amount of money. sometimes it's $50, sometimes $100 and sometimes only $35. this can change depending on what kind of sale they're having in addition to discounting the fabric. eleanor burns recently had a no-shipping deal if you spent over $50 on her fabric frenzy stuff. she has a lot of $4 fabric.
like any other fabric, i prewash everything. i don't trust anything not to shrink and i also don't like working with chemicals on the fabric. that's a personal choice, of course.
hope this answers, more or less, your question.