Second-hand suits sell for a fraction of what comparable used outerwear or knitwear fetches, and many are unsellable at any price. Supply, demand and fit all moved against the category at once.
Supply surged as wardrobes were cleared
As office dress codes relaxed, large numbers of suits stopped being worn and moved into charity shops and resale sites over a fairly short period.
Most were in good condition, because a suit worn to an office a few times a month accumulates very little wear before being retired.
A flood of good-condition stock arriving simultaneously depresses price on its own, before any question of demand is considered.
Demand fell in the same period
The occasions requiring a suit narrowed to weddings, funerals, interviews and a small number of professions, most of which are infrequent.
Infrequent need reduces willingness to search. Someone buying one suit every few years will accept a new one from a shop rather than hunt for a used match.
The buyers who do search are hobbyists looking for particular cloths and makers, which is a small market relative to the volume of stock available.
Fit is nearly impossible to sell remotely
A suit must fit at shoulder, chest, waist, sleeve and trouser simultaneously, and the shoulder is the one dimension a tailor cannot easily change.
That makes the shoulder a hard filter. A jacket with the wrong shoulder width is unwearable regardless of price or condition.
Online listings rarely publish the measurements a buyer needs, and returns on a low-value item cost more than the item is worth, so buyers stay away.
Alteration costs exceed the garment value
A jacket bought cheaply may need sleeves, waist and length adjusted, and trousers hemmed and taken in. The tailoring bill easily exceeds the purchase price.
Buyers do that arithmetic and conclude that a mid-priced new suit fitted once is the simpler transaction, even at a higher headline cost.
Which caps what any used suit can fetch, since its value is bounded by the new price minus the alterations still required.
The exception is at the top of the market
Bespoke and high-end ready-to-wear suits in good cloth retain a genuine resale market, because the cloth and construction cannot be bought new at that price.
Vintage suiting from particular eras also holds value, sold as period clothing rather than as workwear, where fit is judged by different standards.
Everything between those poles trades close to nothing, and a large share of donated suits ends up graded as material rather than resold as clothing.