I bought and sold my first pieces when I was 14, and persuaded Christies to let me do a summer internship at 15.
I sell mostly 20th century pieces but cannot stop myself buying older pieces. I don’t contrive to ‘mix’ but find there are certain pieces that each period does perhaps better than another.
Essentially I have an urban taste, rooted in French classicism, be it 18th or 20th century, and using wonderful objects such as screens or tapestries instead of second rate paintings.
Don’t think an auction is automatically the best value, and don’t think a dealer is intrinsically expensive. If you buy exactly what you need, or truly love, that will prove to be far better value in the long run.