This does not sample people who do not put their numbers in the phone book.
Also, many people work from noon to 2 p.m. so you would be leaving out an important part of the population.
What about people that go to different grocery stores?
Do all of these people have something in common other than the fact that they live in your town?
If so, this is not likely to produce a representative sample.
Not everyone likes to or has to go downtown.
This would be closer to a random sample so it is more likely to produce a representative sample.