Hasbro stomp on 3rd party, unlicensed or unofficial works because they receive no benefit (read: money) from the sale of such things. The other party is profiting from Hasbro's characters and established efforts.
So, how is selling second-hand toys any better? Hasbro don't see a cent from the thousands of dollars that changes hands for old toys - be it Armada, Beast Wars, a MISB Fort Max or Galaxy Shuttle. What is happening is the same, dealers are making money (sometimes, a LOT of money) from Hasbro's intellectual property.
So why allow it? What's the difference between a box of G1 figures, and a handpainted picture of a G1 character, say. Both will change hands for money. Hasbro will not receive any profit. Should toy dealers apply for licenses now, or be banned from selling?
Totally different story. Hasbro already has your money (actually, the money from the store buying their stock) from way back when the 2nd hand toys were first bought. Do you see other companies suing stores that deal in 2nd hand items? No. It's basic economics with the supplier not being a company, but an individual, a middleman as it were. Perfectly legal.
Actually some game companies tried to stop people from re-selling their games. Some suggested making the games kind of a console bound. Meaning that once you put the game inside your console it is registered to that particular console only and basically you couldn't sell it forward. But I think that this didn't quite work. Don't remember all the details, but I guess it was against some laws and such.
Really? Never heard of that story.
My guess is that is to prevent software piracy, as I can imagine it's pretty easy to copy games onto a blank cartridge or disk if you have the right equipment. With us in the Netherlands we're pretty strict on software sales: exchange within 24 hours, or if it has a one-use activation code, no exchange at all.