I see that there are quite a few sellers with records that have 'factory sample not for sale' stickers on them.

Why is it that something that can be put on after should increase the value of a record?

They are of course fakes.

Next time you see one ask the seller if the sticker can be peeled off and see if you get an answer.

I would not buy or sell any record with a peelable sticker. It is far too easy to fake.