Coin collection questions
 

Unless there are rare coins in the collection, it is probably only worth the face value of the coins. Currency does not have to be declared unless it totals $10,000 or more (FinCEN Form 105).

Here is a pictorial guide:
If there are rare coins, you'd probably have to declare their total value if it exceeds $800.

ETA: Proof and Mint sets are also worth more than face value: Modern U.S. Proof Set Values and Prices

You have two sets of laws/rules to be figured out: 1) rules for exporting the coins from Germany and 2) rules for importing the coins into the US.

I have no idea about Germany's laws/rules. You need to contact the proper authorities in Germany (or the German consulate in the US) about that.

In the US there are two sets of laws/rules that will apply: a) US Treasury FinCen rules on coins/currency and b) US Census Bureau for reporting import of commodities/merchandise.

For a) generally the value of the currency (yes, coins are considered "currency") is based on the face value of the coins. So, to be on the safe side, declare if the total face value of the coins adds up to $9,000 or more. This is because although there are exceptions to what currency needs to be included in determining value* you do not want to inadvertently get "caught" out having a combination of your regular travelling money, any Euros and the value of the collection exceeding the declaration threshold nor have the risk of this and any other activity be considered "structuring" either situation could result in seizure.

Depending upon the actual coins, certain coins might not have to be declared as currency but might have to be declared because of precious metal content (eg. gold, silver, platinum coins, medals, bullion).

For b) believe it or not, the Census Bureau is the official repository of information on import and export of commercial merchandise. Tremendous amounts of coin/currency is seized annually for failure to comply with Census Bureau regulations. If your collection can be considered "merchandise" then it must be declared to customs even if no duty is to be collected (duty is not imposed on money but CBP must report to the Census Bureau). It might be considered merchandise if you happen to say you are bringing it home to get it appraised for resale/auction. Even experienced Customs agents can be wrong on the regulations and then your collection gets kept in bond until the situation is straightened out and all the required paperwork is properly submitted. Something have to be done by a licensed broker.

So, I suggest you start doing 3 things now:

1. Have your in-laws start indexing/cataloging the collection now. With that you will then a) be able to determine the total face value of the collection for customs/census declaration and b) be able to use various pricing guides to establish a market value for the collection.

2. check with Germany on any regulations it has regarding exportation of coins.

3. start checking with customs brokers who specialize or have experience in importing from Germany and dealing with coins/currency. If the collection is particularly valuable you might want/need to use their services both for your personal security/comfort and to handle the customs declarations, etc.

* For example, if you had in your possession ten recent series US$100 notes and fifteen US$1,000 notes with a face value totalling US$16,000 you would not have to declare at all even though the face value is over $10,000 and the collector market value of the currency could be well over $100,000.

It's pretty much certain that you won't have $10,000 in the face value of the coins due to their weight ($10,000 in dimes weighs 225kg/500 lbs, more for other coins or mixes).

If you aren't sure you can just declare them at customs and give them a general description ('about 5 lbs of old coins' or '10 mint sets and a bag of other old ones') and then they can decide if it's worth looking at more or not. If you declare them, even if they end up being incredibly valuable, you won't get in trouble even if you guessed wrong as to what they are or their value.