Trigger GrabReference on Opportunity (before insert, before update)  
{   
    Set<String> names = new Set<String>();
	for (Opportunity dp: trigger.new) {
		
		if(dp.FMAV_Selling_Office__c != null) {
		   names.add(dp.FMAV_Selling_Office__c);
		}
	}		
	//queries for  record ID, being the record ID where the dpas.Name field is the same as the dp.picklist field that fired the trigger
	List<Warehouse__c> dpas = [Select Id, Name from Warehouse__c WHERE Name IN :names];
	
	// and if we got back anything...
	if (dpas.size() > 0) {
		Map<String, Id> nameIds = new Map<String, Id>();
		for (Warehouse__c dpa: dpas) {
			nameIds.put(dpa.Name, dpa.Id);
		}
		
		for (Opportunity dp: trigger.new) {
			//set your dp.reference field equal to the returned record's ID
			dp.Selling_Office__c = nameIds.get(dp.Name);
		}
	}
}

@isTest
public with sharing class GrabReferenceTest {


    static testMethod void triggerTest() {
		Warehouse__c testwarehse = new Warehouse__c(Name = 'testWarehse');
		insert testwarehse;
		
		Account testAccount = new Account(Name = 'testAccount');
		insert testAccount;
		
        Opportunity testOpportunity = new Opportunity(
            Name = 'Test Opportunity',
			AccountId = testAccount.Id,
            CloseDate = date.today().addDays(45),         
            StageName = 'Negotiating',
            LocaleSidKey = 'en_US',
            EmailEncodingKey  = 'UTF-8',
			FMAV_Selling_Office__c = 'testWarehse',
            LanguageLocaleKey = 'en_US'
        );
        insert testOpportunity;
		
		Opportunity testOpp = [SELECT Selling_Office__c FROM Opportunity LIMIT 1];
		System.assertEquals(testOpp.Selling_Office__c, testwarehse.Id);
    
	}	

}