Do not sell anything for a month. Almost every regret in this situation comes from moving fast, and nothing in a box is losing value while you learn what it is.
1. Do not clean, polish or repair anything
The single most expensive mistake. Patina, original finish and old labels are often most of the value, and a cleaned coin or a polished brass is worth less than the one you started with.
If something is dirty, photograph it dirty and ask.
2. Photograph and list what is there
Every item, a reference number, where it was. Dull work that pays for itself the moment you are trying to remember which box held what.
Keep anything that came with it: receipts, certificates, letters, old auction catalogues, the original boxes. That paperwork is provenance, and for the valuable pieces it is a large part of the price. What is provenance?
3. Separate the three piles
Ordinary. Most of it. Pleasant, sellable, not remarkable.
Possibly significant. A few things that need an opinion.
Do not touch yet. Anything that might be a genuine antiquity, anything made from a protected animal, anything religious removed from a site. These carry real legal rules rather than policy ones, and the answer is not a blog post. Can I sell genuine antiquities? · Can I sell ivory, shells or feathers?
4. Get a real opinion on the second pile
A museum, a specialist dealer, an auction house. Many will look at photographs for nothing.
Ask two if the answer matters, and be wary of anybody who offers to buy it in the same breath as valuing it.
5. Sell the ordinary pile first
This is the part people get backwards. Selling the common things first teaches you how it works — how to photograph, describe, pack and price — on items where a mistake costs little.
By the time you reach the good pieces you will be competent, and competence is worth more than any advice here. How to open a store
On selling it as one lot
Fast, simple, and usually the lowest price you will get. Sometimes the right answer anyway. Should I sell a collection as one lot or piece by piece?
On storage while you decide
Dry, out of sunlight, off the floor. Paper and textiles especially. A damp garage will do more damage in six months than the previous forty years did.
The thing nobody says
You do not have to sell it, and you do not have to sell it now. If part of it means something, keep that part. The rest will still be sellable next year.
More: should I sell as one lot or piece by piece, can I sell genuine antiquities, how to price for auction.