amazon possibilites

hi guys,

i been dropshipping on ebay for 2 years and am quite up to speed with the pros and cons of this but somethings a miss?

i dont know if i got this totally wrong but i been seeing sellers on amazon itself with £30,000 monthly sales which is impressive, in a nut shell?

as ebay sellers use amazon to fullfill orders on there sales

are amazon sellers selling amazons own products on there own sales channel on amazon ? i cannot believe i havent seen this before now?

are they just putting a mark up on and ordering from amazon as ebay sellers do ?

thanks