I feel your pain in having to downsize. For a quilter you made a hard choise between your sewing machine collection and fabric.....I don't know if I could have done this. However, I do have a couple ideas. First- are you a member of a quilt guild? This is always a good start. Contact a quilting store in the area for other ideas. Then on the shipping thought.
I know USPS GIVES you the boxes, and then you pay postage according to the weight - or there is one that says if it fits it- ships. Find out what size each of these boxes are, take each one home, and fill it to the brim. Take it back to the PO and have them weight it and how much it costs to ship. so you can include this in to price. Then go on Craigs list and advertize Fabric by the Box. You will know how many yards or fabric you can fit into a box and then charge by the Fabric by the BOX! In the beginning limit the selecetions to colors. 1 box of assorted reds = $???.00. and when you get low on supplies, change your add to Grab Bag and reduce the price to sell.
We recently had a white elephant sale at our guild and two ladies were selling fabric. One sold for $1.00 a yard and another sold hers for $2.00 /yd Fat quartes were .50. Good luck.