I have 23 sports team Twitter accounts and have them updated via blogs that are related to each team. This gives the accounts a good source of info each day and keeps the followers happy.
I then throw in some tweets that are blatent ads for products related to the sports team for each Twitter account.
As long as you mix in good info along with your ads then you will never have a problem.
What you sell/promote will depend on the 'theme' of your Twitter account. Do you have spcific Twitter accounts for the items you are looking to sell/promote?