It depends on what you're making,for small objects like lightpulls,eggs or wooden fruit,a cellulose based sanding sealer such as melamine,buffed off with 0000 wire wool then a wax finish buffed off is fine.For bowls which are purely for decoration I tend to spray the same melamine and finish off the lathe with a wax buffed to a sheen.For salad bowls an oil finish is the way to go,I have used danish oil for many salad or fruit bowls or some turners use a nut oil but there can be problems with allergies of course.Morrels will stock Melamine and I tend to thin mine with cellulose thinners .Buy by th e gallon and it will last for ages,buy from turners supplies and you will pay through the nose for small quantities.It all depends on how productive you are and if you are selling your work for profit.As you say demonstrators will be promoting a product they don't pay for,I know I used to be one of them back in the 90,s.Dont fall for the sales patter!