Picture the market as a slope. High up are the expensive things, the smart shops, the careful packaging and the buyers with money to spend. Low down are the cheap things, the plain packaging and the buyers counting what they have. Downmarket points at the low end of that slope. A thing can sit there, or it can move there, and that is the whole word.
The supplied source entry gives the word as down- + market. The picture is already in the parts: market names the scale, and down- names which end of it you mean.
這份資料裡的來源條目說，downmarket 就是 down- 加 market。畫面直接寫在組成部分裡：market 給你那條由貴到便宜的線，down- 指出你講的是哪一端。
Wiktionary entries supplied in this packet (source_refs kaikki:3f6f69a190036ae4:1088830291:3418 and :1088833709:1270): 'From down- + market.'
From the picture
往便宜那一端移動（副詞片語）
同一條斜坡，這次不是位置而是方向。downmarket 在這裡當副詞用，配上走路類的動詞，畫面就是「往坡下走」。兩個片語的差別在於誰在動：一個是自己走下去，一個是被人牽下去，而後者才是帶著批評的那一個。
r · C1
If a shop, company, area, or person goes or moves downmarket, they start selling or buying cheaper goods or services than before.
轉往低價市場
adjective · C1
Made or sold for people who want to pay less: cheaper than other things of the same kind, and usually plainer or lower in quality. It also describes a shop, newspaper, brand, or place that aims at that part of the market.
走低價路線的