That is probably reasonable advice if you are selling to a dealer. I guess they specifically refer to Canadian stamps and modern ones as well. Other countries may be different but I doubt it.
For example on eBay
at the moment Dollar-stamps are offering lots like thishttps://www.ebay.co.uk/itm/Canada-M...OSw6HtdSKuy5
for 60% of face value. Those 6 lots have just ended and I do not think any sold even at that discount
If that is what a seller wants I cannot see any dealer buying it. I am sure that there are many exceptions but as a general rule for collections of single stamps and sets it is probably a good guide.
When you get a package from a dealer through the post covered in Canadian commems and definitives from the 70s and 80s remember that these have been purchased for postage at huge discounts from face let alone from catalogue value such as that lot above.
Have a look at eBay
lots - not "buy it now
" lots - which actually sell and you will get some idea of what the market is like and that is selling to collectors !!.
But remember this is all very general