No that's a great question.
A sign in front of your business or on the side of a caravan should be a must have, even if it doesn't have any words. For example a hanging sign of an anvil or weapon would be enough for a blacksmith, or scales for a common merchant.
But I think advertisment away from the store front should be at most a flyer on a public notice board, I doubt most towns or city's would be ok with people spamming flyers all willy nilly.
The next best thing would be making friends with local shop owners or important locals seeing if they can post a note for your business in theirs. The best option (in my biased opinion) would be to make friends with the local tavern/inn owners and have them recommend you to anyone that asks them, since that'll be the first place travellers will go.
And of course after you get settled if your goods are of high quality or you have good deals your products will sell themselves. Especially if you are the only person in town doing a specific trade.