The word "seller" is used when describing a salesperson, but "bestseller" is used when describing an item (usually a book) that is being SOLD in large quantities.
Could you please explain this difference?
"Seller" - is it an active adjective (someone who sells) or a passive adjective (item that is being sold)?
Seller is a noun. It describes a person selling something. "The seller wanted far too much for the car."
In informal English, seller is also a noun and is used in, e.g. "This car is a seller, not a keeper." = "This car is {one that you should sell - a car to be sold} rather than keep (a car to be kept.)"
In "This book is a bestseller" "bestseller" is also a noun. It derives from the middle voice (or mediopassive) of the verb to sell.
The middle voice is distinguished by its having no agent:
"This book sells well/best." is an example of the middle voice or mediopassive. It is obvious that the book does not "sell" anything and who or what is doing the selling is unstated.
Hence "This book is a bestseller."
In this case, "seller" is a noun and is joined with "best" to form a new common noun. (See the informal car example above.)
We have to distinguish two meanings for the noun seller: a person who sells and a thing to be sold.
Bestseller derives from a thing to be sold, as in Make two piles of your books: the sellers and the keepers and, ultimately, the intransitive, passive sense of the verb sell, as in Her new book sells for $34. and The new toy is selling like hotcakes.
seller (n.)
1.a. One who sells.
2. A thing to be sold.
1831 The packs, or shepherds' lambs, are divided into two sorts, sellers and keepers.
P. Sellar, County of Sutherland 80 in Farm-rep.
bestseller (n.)
An item that is sold in greater numbers than anything else of its type; esp. a book whose sales are the highest of any book in a particular category or during a particular period. Also more loosely: something that sells exceptionally well.
Formed within English, by compounding.
best adj. + seller n., after bestselling adj.
bestselling (adj.)
That achieves better sales, or is sold in greater numbers, than anything or anyone else; that is a bestseller.
Formed within English, by compounding. Etymons: best adv., selling adj.
sell (v.)
6.a. intransitive in passive sense. Of a commodity: To find purchasers. to sell for, at = to fetch (a price).
1833 They sell at about a shilling a dozen.
H. Martineau, Brooke & Brooke Farm (ed. 3) v. 631896 This, madame,..is selling very well.
H. G. Wells, Wheels of Chance i. 7[OED online]
Note, however, that the only adjective here is bestselling.
We can also find sell as a noun:
sell (n.)
Something to be sold or caused to be accepted
The new mystery novel was an easy sell
also : someone to whom something is sold
The new purchasing agent was a tough sell
M-W