My experience is limited to wine stoppers. I've made quite a few - I don't sell them, but I always include a stopper with a bottle of wine that we give as a gift to the host when we are invited to parties. And our neighborhood has lots of parties!
Stoppers generally are spindles with the grain parallel to the axis of the lathe, and threading a mounting hole in end-grain is a problem. Yes, it can be done, but I've pretty much standardized on making wooden plug inserts with threaded holes. I make them in bulk (production mode) by drilling 5/16" holes in a piece of "1-by" face-grain timber (which is actually 3/4" thick), threading the holes, cutting them apart, and then mounting them on a stopper mandrel to turn each plug to a 5/8" diameter.
To make a stopper, I mount the blank between centers, turn to round, and put a small tenon on what will become the top. Then, I grip the tenon in a chuck to face off the bottom. I prefer to either make the bottom concave, or else cut a mortise in the bottom with a forstner bit, so that the seam between the finished turning and the stopper cannot be seen when viewed from the side. I know this isn't necessary, but it's just what I do. Then I drill a 5/8" hole, insert and glue in one of the threaded plugs, and after the glue sets, finish off the bottom. I always apply finish/sealer on the bottom so that wine cannot wick through the fibers of the wood and stain the finished stopper.
Then, I remount the blank on a threaded stopper mandrel to finish turning, and apply a finish. The only thing I can say about turning is to avoid a pointed top - in use, it is necessary to press the stopper into the bottle, and a pointed top makes that process painful. So I always have some kind of rounded top. Beyond that, it's a matter of using your esthetic sense to decide what looks good. I tend to prefer stoppers that are not excessively tall, but that's purely a matter of personal taste.
I use either stainless steel stoppers with threaded mounting studs (I prefer Ruth Niles' stoppers) or the PSI silicone sleeve/dowel stoppers. There's a big difference in price, but functionally they work just the same. Ruth's stoppers just screw into the threaded hole in the insert that I glue into the turning. With the silicon sleeve design, the 3/8"x16tpi threaded hole has to be enlarged to accept a 3/8" dowel - I do that by gripping a drill bit in a bench vise, and using it to ream out the threaded hole in the bottom of the turning. This is not difficult since reaming amounts to little more than removing the interior (female) threads. Then, I glue the dowel that holds the silicone sleeve into the enlarged hole.
For finishes, I have used WOP, a shop-made long-oil varnish, and a wiping lacquer. I would avoid shellac-based friction polish since alcohol could attack the finish.
Most of my stoppers are simply turned spindles. But I have experimented with two interesting variations. One is to include a captive ring on the spindle. That always triggers the 'how did you do that?" question. By the way, finishing a stopper with a captive ring is not easy - I generally use a simple oil finish on them. Another is to do a spiral cove on the spindle. Both are fun for me to do, but they do involve a lot of work so they may not make sense if you are making stoppers to sell.
