I want to categorize transactions as sales or purchases in BUSY. What is the right way to do this?

You can categorize the sales and purchases in BUSY by creating multiple sales and purchase account then can tag in item master, item group, material center and sales/purchase type. 
 1.Create the sales and purchases account. 
 Step 1: Go to Administration section then click on master menu 
 Step 2: Select the account option then click on add 
 Step 3: Create the account of sales with sales group and purchase with purchase group account. 
 2.To made configuration for tagging the sales and purchase account in item master or item group or material center or sales/purchase type. 
 Step 1: Go to Administration section then click on configuration menu 
 Step 2: Choose the features and option then select inventory tab 
 Step 3: Now, select the required option in tag sales/purchases acc. with field and save the configuration. 
 3.To tag the sales and purchase account. 
 Step 1: Go to Administration section then click on master menu 
 Step 2: Choose item master or item group or material center or sales/purchase type as per the selection in inventory tab then specify the sales and purchase account. Afterward, data will appear in profit and loss as per the tagging.