My questions was what do you say to the spectators if they ask you why you're using coins no longer in circulation. What's your "excuse" for using it.
This question is probably best answered by the premise of the coin effect.
Take for example the classic effect from Bobo's called "The Gadabout Coins" (page 182 of "New Modern Coin Magic") or any of its more modern variants (such as Dan Watkin's "Sticky Coins".)
Three coins are introduced, two go in the left hand and one is placed in the pocket. When the left hand is opened it contains all three coins, one having apparently traveled from the pocket to the hand. This is repeated three times, then the coins vanish leaving both hand entirely empty.
Is the magic occurring because
you have special powers and are causing it to happen, or because of some peculiar quality of the coins themselves?
If you are lucky enough to find some matching Barber halves in junk silver at a local coin shop, you could try to get matching dates. Like for example 1902. Before bringing out the coins you could ask, "Have you ever seen a half dollar? How about one that is over 100 years old?" If there is any interest in seeing such a thing, bring out the coins and have them examined. Call attention to the dates and say something along the lines of, "All of them are exactly 106 years old... And they have been together all that time, can you believe that? When I found them they were together and every time I try to separate them (place one in the pocket) this happens (open left to show one has jumped back)." Etc, etc. "I even thought I lost them once (final vanish) but I was relieved when I found that they all had jumped back into my pocket!"
This is clearly a case where common, "modern" coins would not generate as much interest.
Maybe it isn't even the coins at all. Find a "vintage" sports jacket that looks like someone shot and skinned a couch. Wear it around until someone calls attention to it, then say, "Yeah, horrible isn't it? Looks like someone shot a couch!" When they ask why you are wearing it, say "The lady at the thrift shop said that it use to belong to a local magician... I didn't believe her at first, but I tried it on in the dressing room and noticed that every time I pull on the material, this happens! (produce a coin from the cloth, show it and then draw attention to the date.) "Look! This coin is over 100 years old! (or from the 40's if you are using walking liberties, etc.) Keep producing coins (flurry style) and putting them in your pocket. After the fourth of fifth coin, pause for a second like your thinking and then say, "Hey, do you know where a person can sell silver?" Before they can answer, glance into the breast pocket and then start pulling out a long rainbow colored paper streamer that keeps going and going and going (mouth coil)...
The last example was just plain silly, but you get the idea.
Create a premise which answers the "why" before you are even asked.
Hope this helps,
-ThisOneGoesTo11