If you can determine what country each coin is from then take a trip there and spend them. But they might be demonitized (no longer used)
I recently attended a coin show, walked up to a table which included a pick box of world coins, and asked the dealer if I could add about 60 coins to his box. He said 'yep!', I said 'awesome!', dumped the coins, and walked away with a smile. It was a veritable "Win-Win" in my book. Most were coins I had bought (overpaid) as a child from Littleton. I would have listed them on eBay, but it's just not worth the time. Sadly. :-/
I'm at that age where when giving away stuff I say "Thanks for taking that off my hands." Your coin dealer story reminds me of the time I was in a used book store and a little boy brought in a wagon-full of books. Dealer says "$5 for the whole lot, $10 just for this one book." Dealer explains: "I am in effect charging you $5 for me to take all those other books out to my dumpster."
Donate foreign coins to your local boy scouts. Their members can use them to earn the coin collecting merit badge. Helps them and maybe starts a new coin collector who will grow up and want to buy your coins from you or your estate.
I keep any coins or currency, foreign or not. I've traveled the world and always brought back coins and currency as a memento. I usually kept the ones that had what I considered to be the best artwork and/or historical depictions. I'm going through all of mine right now and found that I had several silver coins and a couple of really old coins worth good money, according to NGC current values. If you don't want to spend the time to find the worth of your coins, then Harley's idea of giving them to the Boy Scouts is a good one.
There is a drug store in our town that has a free bookrack in their lobby that is very busy with customers picking up and depositing used books. I shed about five years worth of collector and antique magazines from the '70s about six at a time over a period of months until they were all gone.
I have an old cookie can that I've been putting them in for about 35 years. There are around 1,500 coins in that can now.
Coins are so small unless you have an insane amount they won’t take up very much room. Personally I would just keep them, no loss there!
