I've been trying to create a trigger to connect entries between two custom objects upon bulk upload...I was able to get some help with the code but there's a compile error that I can't figure out. Can anyone help? It'd be greatly appreciated.

What we're trying to do:  basically we have two custom objects, "Sales" and "Reps". We upload a list of sales (in .csv format) into the Sales object each week. We want to make it so that every time a Sale entry gets created it is assigned to the Rep who made that sale (Sales has a master-detail relationship with Reps). Each sale lists the Name and ID# of the rep who made it, but not the Salesforce ID of the corresponding Rep entry. So we want the apex trigger to match each Sale to the correct Rep by comparing the Name and/or ID fields in both objects.

The code we've got so far is:

trigger SaleTrigger on Sale__c (before insert){

   //This set will store all the RepId's.

   Set<String> repIDs = new Set<String>();

   //Loop through all the new sales to be created.
   for(Sale__c sale : Trigger.new){
      //Get the rep's ID of each sale and add it to the Set
      //Note: I am assuming the API/Unique name of the Rep ID     
      //field is Rep_Id__c;
      repIDs.add(sale.Rep_ID__c);
   }

   //Now we have all the Rep ID's in the set.
   // Issue an single SOQL to get the Salesforce ID's of all the reps.
   
   Map<String,ID> mapReps = new Map<String,ID>();
   for(Rep__c repRecord : [Select ID,Rep_ID__c From Rep__c WHERE Rep_ID__c IN (:repIDs)]){
      mapReps.put(repRecord.Rep_ID__c,repRecord.ID);
   }
   
  //The mapReps has a map of all Rep ID's with Rep Salesforce ID's
   for(Sale__c sale : Trigger.new){
      ID repSalesforceID = mapReps.get(sale.Rep_ID__c);
      //This is the MD field in Sale__c.
      sale.Rep__c = repSalesforceID;
   }
   
}

The error I'm getting on trying to save is:  "Invalid bind expression type of SET<String> for column of type String"

It occurs on line 19 which is: 

for(Rep__c repRecord : [Select ID,Rep_ID__c From Rep__c WHERE Rep_ID__c IN (:repIDs)]){

Can anyone help me figure this out? I'd be extremely grateful.