When it comes down to marketing in any form whether a mulitnational business or a sole trader, it is important to get inside the mind of the customer / potential customer.
What would you do in there circumstances if you were looking for a tradesmen? Personally I would look in the yellow pages or online, and looking in the local supermarket wouldnt even cross my mind.
When you go into the store take a few numbers down of the tradesmen who are currently on the board and see what business they have gained from the advertisements.
As i have said before the most important thing is to monitor youre marketing. Ask everyone who calls you for a quote where they got your number from. If possible keep track of how much you have earnt from each form of advertising vs how much you have spent on it.
Neil