Define "Top Seller"

It means it has been sold most(i. e. more than the rest of the games) in a given time period. Seriously people, get a dictionary

It's crap. They define it just as it pleases them to advertise. Even crappy games call themselves "top sellers".

Number of transactions in the past X day(s). Thats all it means.
Which is why its always riddled with titles that are on sale, as during a good sale those titles get picked up significantly more.

that's easy. You just have to give Gaben a big sack of money and he'll put your game into Top-Seller column.

They define it by most $ sold. If a game sell for $10 and other sell for $30, but you sell 3 copies of the $10 one and none of the $30 one for a period, the $10 one will be ahead as top seller. If you sell a overpriced pack of dlc from a f2p for $100, then that will go to top over the $10 they sold 3 times.