In some cased, buying a used sewing machine is a good idea. You can usually get “more bang for your buck” this way. I’ve seen used sewing machines go for a a third (or a few hundred dollars) less than the comparable new one. In some cases though, either a used machine is not available, or the savings are not enough to warrant buying it.
It helps if you have an idea of what type of work you will be doing. If this is just for home use, buying a used industrial sewing machine doesn’t make sense for example.
Consider showing the seller some of what patterns you will be using, and ask for some references.
Watch out though: make sure you trust who you are buying from. Ask to see maintenance records or usage patterns for the sewing machine before you buy it. What kind of warranty will you be getting on the machine, and what is the return policy?