Why do sellers overcharge on P+P?
If they start an auction at 1p and charge the correct postage then they are going to lose money,so they will charge a P+P price to cover fees ,product costs and a profit
this is due to ebay fees system,
ebay should charge a fee based on overall price rather than selling price
if you sell buy it no 99p and £2.99 P+P the fees are 10p
if you sell £2.99 plus 99p P+P the fees are around 30p
20p is not a lot but its 3 times as much and over a month high volume sellers dont want to pay 3 times the bill they get.
and if seller sells at 99p it attracts more buyers and generates more sales ,so ebay probably get more overall in fees due to this