Build coherence beats raw levels
An account is worth what it can do, and that depends on whether its levels work together. A spread of mid levels across everything is worth less than a focused build that can actually enter the content it was made for, even when the total level is lower.
This is most visible on pures, where a single unwanted defence level can move an account out of its bracket and take a large share of the value with it. It is true more broadly though: the question a buyer is really asking is "what can I do with this tomorrow", and scattered levels answer it badly.
Unlocks are the real inventory
Quest completion, achievement diaries and the access they grant are frequently worth more than the levels around them, because they cannot be bought and they gate content directly. Recipe for Disaster is the standard example — barrows gloves are cheap, the chain behind them is not.
On ironman accounts this dominates the valuation entirely, because nothing in the bank arrived by trade. There, the drop log and the diary tab are the product, and total level is almost incidental.
Provenance is a price factor, not a footnote
Two identical accounts are worth different amounts if one comes with its original registration email and the other does not. The one without it carries recovery risk that never fully goes away, and a buyer who understands that will pay less — correctly.
Sale history works the same way. An account that has been flipped repeatedly has accumulated more people who might recover it. Sellers who can say honestly that they built the account and hold the original email should say so, because it is worth money.
What buyers routinely overpay for
Bank value on a normal account is the big one. Anything tradeable can be bought separately, so paying a premium for a stocked bank on a tradeable account is usually paying twice — once in the account price and once in the opportunity to have bought the items directly.
Cosmetics and rare-ish untradeables that do not unlock anything are the other. They are worth what a particular buyer feels about them, which is a thin market and a poor basis for a price.
The reverse also happens: unusual, hard-to-replace unlocks are routinely undervalued by sellers pricing on total level, which is where a patient buyer finds value.
The short version
Price the unlocks and the ownership provenance. Total level and bank value are the two numbers most likely to mislead you in either direction.