I know that this is an unpleasant thought BUT unfortunately it's one that SHOULD BE DONE. Let's look at the 2 different categories.

Dealer - Insurance for dealers can usually be done through regular inventory insurance. Each company handles it a little differently, BUT make sure you have the transportation clause in your policy. This insures the merchandise while being shipped and ELIMINATES the need to put insurance on any packages that you ship out.

Collector - If you don't SELL posters, you can cover your collection with a rider on your home insurance policy, called a floater. Insurance companies have problems placing a value on a collection AFTER it's lost so it's VERY IMPORTANT… make a list of your collection. Put a value on each poster, preferably from one of the price guides where you have documentation of the value of the poster. (that's the fun part). It's actually better to even take a photo of each to attach to the list. If you have a very large or very valuable collection, it's better to get an actual appraisal. Submit the list to your insurance agent and keep 1 or 2 copies in a safe place.

As your collection changes, periodically change and resubmit your list. Some companies only want you to keep an updated list and proof and don't need a copy for themselves. The insurance policy then covers them as personal property.