"I'm running out of storage space ... is rolling up a canvas okay or will my paintings be damaged?" -- Jos
"Someone's bought a painting from my website, can I roll it up to post it to them, because it'll be a lot cheaper to post?" -- Mike
A painting ought to survive being rolled up and posted, provided you ensure the paint is completely dry and don't roll it up too tightly. But realize that it does have risks associated with it, starting with the potential for damaging the painting when you take the canvas off its stretchers and when someone re-stretches it. As for storing painting rolled up, it's not an ideal long-term choice, so perhaps limit it to your 'B' grade paintings if you don't foresee having additional space at any stage.
How to Roll Up a Canvas
To minimize the risk of damage, roll up the canvas as loosely as possible with the paint on the outside. If you roll it up with the paint on the inside, the paint may wrinkle up (especially if it's thickly applied or got a lot of texture).
If you're skeptical about this, do this quick test: bend a finger and see what your skin does -- on the outer edge it stretches slightly to cope with the curve, whereas on the inside it folds up against inside. Paint does the same, albeit not as visibly.
Don't roll the painting up tightly; you want it to be as loose or big a roll as possible. If you're putting the painting into a tube for posting, buy a large-diameter one. Ideally buy two tubes, one to roll the canvas around so it can't accidentally get squashed, and another to put the rolled-up painting into.
Whether or not you put something over the painting before you roll it up is debatable; you want to protect the painting but you don't want something that'll stick to it, rub off on it, or chafe it. Something with texture, such as bubble wrap, may 'imprint' itself on the paint (called ferrotyping). Paper may absorb moisture and go moldy, especially thin tissue paper. Plastic that's too thin may adhere to the painting like clingfilm and be hard to remove. If you use another piece of canvas, make sure it has a fine weave not a coarse one.
But then you also don't want a rolled-up painting to chafe against the inside of the tube, so you do want to put some sort of packaging between it and the tube. (Resist the temptation to roll the canvas up with the painting on the inside to solve this problem.) Your best options are either a stiffish piece of plastic (like the plastic sheet you'd cover the floor for decorating) or a spare bit of unpainted canvas. Make sure it doesn't have dust on it, or wrinkles/creases in it.
How Long Can You Store a Painting Rolled Up?
Well, in an ideal world, you would do it for as short a period as possible. If possible, store a rolled canvas vertically rather than horizontal; that way its weight is on the outer edge of the canvas not lying on side of the painting. See if you can't find a spot to store the canvas unrolled and flat, but don't store too many on top of one another as the bottom one will eventually get squashed by the weight. Unroll a painting at room temperature, not when it's cold and the paint relatively rigid.
How Dry Must the Paint Be?
Utterly and totally dry, not just touch dry on the surface. Don't be tempted to roll a painting up when it's more or less dry can arise, particularly with oil paints. Not even if you've a buyer who can't wait for a painting... Rather risk losing the sale by telling the person to wait a bit, than end up with a dissatisfied client in possession of a messed-up painting.
How to Get a Canvas Painting off Stretchers
Obviously, you need to remove the staples or nails that hold the canvas onto the stretchers. Remember, you don't want to tear or rip the edges of the canvas as it'll be needed when the canvas is re-stretched. So be patient as you try to lever out the staples. If you haven't got a suitable woodworking tool (e.g. long-nose pliers), rather try a flat screwdriver than something sharp such as a pair of scissors. Don't cut the canvas off the stretcher!
How to Get a Canvas Painting Back on New Stretchers
You do it more or less as you would a blank canvas, folding the edges over the stretchers and stapling. Just do it far more carefully and gently!
Warn a Buyer to Expect a Rolled Canvas
If you're going roll up paintings you've sold to post them, best you warn buyers up front. Most people would expect to be able to hang a painting on their wall immediately, and probably have no idea how to restretch a canvas. (An experienced picture framers should be able to do this.)
For an indepth look at how to safely package an acrylic painting for transportation, including diagrams of making packing cases, read Just Paint Issue 11, published by Golden Artist Colors. (Count how many times it gets mentioned the paint needs to be dry!)