It takes a bit of work to find the median but I'll try and walk you through it. Duncanver asked for the median for postal entire so let's do that. First go to the
ebay category for "Stamps" and search for "entire"
This shows that there are 5357 entires for sale on
ebay. Next go to the criteria column and choose "sold"

Now it shows 4307 entires have been sold on
ebay, this number is for the last 90 days.

To find the median we have to find the price at which 2153 were sold below the price and 2154 were sold above the price.
I go back to the criteria field and for starters use 5;

This shows that 1716 were sold with a price greater than $5 so obviously this is too high, I now try a few lower numbers until I find the one that is close to the median


I got pretty close using $2.70 which shows 2135 covers sold over this price the right answer is slightly less but I ran out of time.

I'm not sure how significant the median sale price is but it does indicate that a LOT of low price items are sold on
ebay. I tried it for Unused 19th C US stamps thinking this should be pretty high and it was higher, about $10, but not as high as I thought.