I do varnish my oil paintings. One unvarnished painting I bought on DPW arrived slightly scratched by a tiny sliver of cardboard that was sandwiched between the painting the packaging. It doesn't take much to damage an unvarnished painting.
Really, varnishing is no extra effort in comparison with the time it takes to actually the paint the piece or even the time it takes to package the painting up for shipping. With a little practice it is very quick to do. I varnish 3 or paintings or so at once. Also, a lot of buyers probably would not have the confidence to varnish the painting themselves.
I recommend this podcast: - an interview with Robert Gamblin, founder of Gamblin paints. The whole interview is fascinating, but if you skip to 1 hour 14 minutes, he talks a little bit about varnishing.
Understanding oil paints with Robert Gamblin.
I use Gamvar original gloss and apply a thin, even coat using a Princeton #10 flat brush. I keep that brush separate, labelled 'VARNISH ONLY' and then clean it with OMS. I've found that it is important to leave the painting completely flat (horizontal) for at least the first 12 hours to ensure an even finish.
Joe, I agree, I'm not impressed with the new satin version of Gamvar at all. I tested it on an old painting and found it left visible brush marks. I haven't found that problem with the original clear gloss Gamvar.