it depends on their condition
Contributions of clothing and household items.
A deduction for these contributions will be allowed only if the items are in good used condition or better.
However, this rule doesn't apply to a contribution of any single item for which a deduction of more than $500 is
claimed and for which you include a qualified appraisal and Form 8283 with your tax return.
the sites you mention are sellers. their price is probably higher than true market value, you are not selling the items you're donating them.
