if the value is not known, should a real estate salesperson list a
house for sale for what the seller wants for the property of should
he/she research first to find a value?

I'm not sure that a value is ever "known".  Part of the "value add" of
working with a realtor, would be having them pull "comps" so that you
get an idea of the price range.  The property ultimately belongs to
the seller though, so they have last say in price.  If they ask too
much the house either won't sell or could take a long time to sell. 
If they ask too little, you may be leaving money on the table (meaning
the buyer(s) may have been whiling to pay more).