The short answer is always yes.
The long answer is generally maybe.
The blunt answer is unfortunately, sometimes, no.
Yes, you can make and sell terrain in all sorts, periods, scales, materials and price points. There are many companies that specialize in that and a good number of them do it full time as their day job and haven't managed to starve to death yet.
Whether or not you can get into a competitive market depends on the quality, price and designs which you hope to sell. You need to set yourself apart from a few dozen straight retail companies and maybe a dozen or so custom companies who are out there right now...and that doesn't even include the few hundred weekend warriors who sell items they make in their free time either on eBay, through webstores or on direct commission work.
With the price points, you have paper models on the low end of the spectrum coming from companies like World Works, slightly above that are die cut card. Then you move into laser cut wood, vacuum formed plastics, plaster castings, injection molded plastics, resin and finally custom work (with custom sometimes being cheaper than resin depending on the size and scope of the product). Within all of that you have everything from 3mm scale all the way up to 1/35 scale or so and from Ancient Persians all the way through to the 41st millennia or however you want to look at that.
That isn't to say that you don't have gaps in the market - there are plenty. You can also do what other people are doing, just do it better (cheaper, faster, better...).
Occaisionally though, you just have to be realistic (or be told by someone who is being realistic) and the answer might be no, or at least not yet. Some very enthusiastic people decide to start making terrain (or some other aspect of the hobby like commission painting) and they are deficient in some way when it comes to doing commercial work. They might not be fast enough, skilled enough, priced right for the level of detail they produce or they might just be in the wrong location. Sometimes they are things which they can solve through practice over time or by improving their processes to lower costs. Other times it is out of their hands and there is no reasonable way to deal with the problem.