'Retail' value has little purpose unless you are planning to become a full-time dealer. Retail values can be achieved by folks who have invested big money into their businesses and need to cover their overhead.
Market value, which is often a fraction of the retail value, is what most of us get when we try to sell our stamps. Not only is this generally closer to 'wholesale' value, but it also encompasses liquidity. Like all other collectibles, the advent of the internet has greatly impacted market values. Suddenly the 'supply side' exploded with offerings as every hobbyists turned into 'part-time' sellers.
To better understand the true market value, you might start with eBay
'sold' search function. Don't look at eBay
'for sale' listings, folks can ask whatever they want. Filter the eBay
search for 'completed' and 'sold' listings. Additional discovery can be done for other online auction sites to get an even broader understanding.