I don't do it because I've been disappointed by so many swatches online. You see something you think is nice, get a swatch, and it turns out the color isn't right (so many sellers don't bother to color correct images); the pattern isn't the scale you assumed; or the hand isn't what you expected.
I find it helps to buy from people who can give you more information. Ed here is good -- you can ask him if he thinks the fabric he's selling might be good for a sport coat, for example. I also find his colors are accurate in the photos. Same with Alden at LL, where you can ask him if the fabric is spongey, densely woven, etc. I've never been disappointed by the things I've bought from them, and they've all been bought online (i.e. using just word and photos).
A random seller on eBay might not give you that kind of reliable info. Or they might not have the same perspective as a clothing enthusiast.