Some sellers charge above costs for one reason or another, but $4-5 is unrealistic if the seller is packaging and posting them as a parcel. If they are, it is $6.95 postage for up to 500gm, and (if using a flat rate satchel) around $11 or so for up to 3kg, plus any other costs like packaging. They may also be using flat rate boxes (also parcel rates), so in that instance the pricing would be justified. If you look at the postage and payments tab, it should tell you which kind of delivery service the seller has selected.

$12 for a single game/dvd being posted as an unregistered large letter does seem a bit steep, but there are postage caps for flat rate in most media categories already, however because some services can cost more than that, there is an option to use calculated postage for services like the satchels / parcels.

Best thing to do if you don't like the prices is to not buy. 😉