Yes, silencers are legal in 38 states including Maine. You don't need a license to buy a silencer but you have to pay a one-time $200 tax for each silencer you buy. Also, you must live in a state where it is legal to own a silencer and you must meet certain criteria to be able to own one.
Silencers are regulated by federal law and on a per-state basis. The National Firearms Act (NFA) branch of the ATF controls any transfers and all silencers are registered by this branch.
If you want to buy a silencer, you must be fingerprinted and the chief law enforcement agency where you live will have to sign your application. You may obtain an application to buy a silencer and a fingerprinting card from HDS Sports. Your application must be sent to the ATF for approval and this process will take between 3 - 5 months.