Used or Ex display products
In almost every case it's possible to view the product before buying. Depending on the type of the product, this will be at the home of the seller or the showroom of the trade seller. If you are unable to travel, it may be possible to arrange a video call.
If a product has already been removed and is in storage, this information will be in the description of the listing.
New and New (other) products
New products are most often delivered in their original packaging, therefore, viewing availability will have to be discussed and agreed on an individual basis.