Most (but not all) standard sized UK DVDs are Factory Sealed by the manufacturer.  The differences between a factory sealed dvd and a re-sealed one are quite hard to explain verbally. To me, the cellophane wrapping for a factory sealed DVD looks a bit like a neatly wrapped present in the way that the corners are folded in and overlapped along the top and bottom. DVDs that have been re-sealed usually have a thin nobly edge all around the seal. The plastic sleeve of the DVD case is also very often slightly 'bumpy' - this is because most re-sealed DVDs are heat sealed which unfortunately can affect the plastic sleeve (i.e. the heat starts to melt them!). 

(Two sealed dvd cases: Most Factory-Sealed DVDs look like the bottom one)

The main problem with re-sealed dvds is that they are usually returned shop stock. A lot of  physical shops remove dvds from their cases to prevent theft. If the shop later returns the dvd to the wholesaler it is then re-sealed by them. Technically it is a new dvd - after all it has never been played, and if you went into the physical shop that is the condition you would get it in. Thankfully the overall number of re-sealed dvds is relatively low, but in my experience about 50% of those dvds which have been re-sealed will have the handling marks (maybe just a thumb-print, sometimes a few light scratches) of the sales assistant of the shop that originally stocked it. That is why I choose not to sell dvds which have been re-sealed. It is not the sellers fault that a dvd has been re-sealed - they do come from the wholesalers like that, but most DVD sellers handle enough DVDs to know the difference between a factory sealed dvd and a re-sealed one.  Many choose to ignore the fact that the dvd has been re-sealed and hope for the best. Some sellers are unable to state the exact condition of the dvd because they don't actually have it in the first place (they won't order it from their supplier until they have received your payment). I am a bit fussy as a consumer, and therefore also as a seller. I only sell DVDs that I actually have in stock. I only list dvds as 'Sealed' that are genuinely factory sealed. If I list a DVD as 'New' it will be absolutely brand new, in mint condition.  Anything less is sold as 'Used'.