The market for ordinary 20c catalogue valued stamps is down.
The market for good condition stamps is always up. Collectors want good stuff and will pay a premium for it.
So, probably most of your stamps are so-so condition and, if so, you should expect a so-so price.
To sell at a loss compared to catalogue value is to be expected and is ordinary. Except only for high valued (catalogue) and also in great shape stamps.
The more you sell at one time the lower the percentage of catlogue value you should expect.
Sel;ling stamps one by one expect to sell 10% to 15% of them at a time, per month. Keeping them listed up on a site or available to collectors somehow and always advertised costs a price also.
Your time spent and involved is also a big factor.
Prices sold for on
ebay, etc are generally a god idea of what regular good condition stamps sell for, But not any indication of what extraordinary condition stamps will sell for. Poor condition stamps will sell for 10% of a good condition stamp, except to beginners perhaps.
Collecting specialties will almost make a collector pay extra for that stamp or cancel he really wants. A nice complete set of stamps in worth a bit more also, if advertised as such.
Details, details.