Is it legal to sell old toy cap guns or replica guns? 

The answer is Yes, but there is something you need to know first.

In 1988 the Federal Government passed a law that states It shall be unlawful for any person to manufacture, enter into commerce, ship, transport, or receive any toy, look-alike, or imitation firearm  unless such firearm contains, or has affixed to it, a marking approved by the Secretary of Commerce, as provided in subsection (b) of this section.

But the law goes on to say each toy, look-alike, or imitation firearm shall have as an integral part, permanently affixed, a blaze orange plug inserted in the barrel of such toy, look-alike, or imitation firearm. Such plug shall be recessed no more than 6 millimeters from the muzzle end of the barrel of such firearm.

The key to the law here is permanently affixed blaze orange plug inserted in the barrel of such toy.

Any toy gun manufactured after this bill was passed had a blaze orange plug built into the gun. So what about my cap gun from 1950's? Can I sell it on eBay? The answer is YES, but you will have to permanently affix a blaze orange plug to the barrel. Remember that you'll need the plugs anywhere you sell them (not just eBay). 

Now the big questions are: What is a blaze orange plug?  Where do I get them? How do I install them?

As far as I can tell, a plug can be made plastic, wood, or foam.

Here are some common items that are found around the house. See photo below or click here to Enlarge Photos.

My personal choice are the Nerf Darts. They extend far into the barrel. I use a hot glue gun to hold them in place.

Please note: 

These are USA laws.

If you are listing a Toy Cap Gun on eBay, all of your photos must show the whole gun and plug. No close-up photos showing details.

Auctions must be listed for USA only.

Never send a Toy Gun outside of the USA. People outside the USA must refer to the laws of your own country.