These look like the type of RETS field names that come from Interealty - one of the major RETS vendors. There are five variations of RETS field names - the "SystemName", "StandardName", "LongName", "ShortName", and "DBName." You appear to be querying on the DBName, which usually is not queryable. Try the SystemName, which should always be queryable. Interealty uses numeric SystemNames, so your list price field, ListPr_176, would very likely have the SystemName "176". The part of the query for the price would then look more like "(176=0+)". The entire query code should probably look more like this:
$rets_modtimestamp_field = "260";
$previous_start_time = "2013-01-01T00:00:00";
$listing_status = "383";
$listing_price = "276";
$query = "(".$listing_price."=0+),(".$listing_status."=|A),(".$rets_modtimestamp_field."=".$previous_start_time."+)";
(I added a presumed lookup value for Listing Status of active)