I don't think there are many "physical markets" these days. When someone goes to the market to shop, the only thing they can buy in most developed countries is vegetables and maybe a few other foods. So we would say "he sells vegetables at the market". While there are probably countries that still have "physical markets" that sell more than just vegetables, most developed countries now have retail stores or shopping malls or strip malls or department stores, rather than "markets" (at least in North America and so far as I know, Western Europe). As for national markets, "the company plans to enter the US market" is good and "the company has been working on entering the US market" is good, but "working on a market" suggests trying to fix it, as was pointed out earlier. Finally, people can work in markets (e.g. Susan works in the US tech industry") but I don't think we would say "a company works in the US tech industry". We would say "Company ABS sells in the US" or "participates in the US tech sector" or "sells tech products in the US market" or any of many other verb/preposition combinations. Headlines are so much easier than prepositions! I hope that it helpful.

Hi Sasha, Sufia gave you a perfect answer. To work on something means it doesn't work and you're trying to fix it or you're solving a problem/putting a report together/finishing an essay. For example, I've been working on my essay in the last few weeks.

In the second sentence, you say in if you intend to say they have been operational. So they have been working in the market... If you say working on the market, it conveys there was something wrong and someone is trying to fix it