How disappointing!
First of all try contacting the seller and politely explaining that you are a collector and that unfortunately only a few of those ribbons are genuine MLP. You are only interested in genuine ones and also 20 were listed for sale but you have only received 18
(you are sure on this and there are no clear pics of all the ribbons on the auction? and there are definitely 20 in the pic or are they just in a pile on there? - just checking BTW before you go any further with sorting out the problem )
Give the seller a chance to respond, explain, and offer a solution.
If the seller doesn't respond, will not offer a solution etc then you can open an "item not as described" case. You should explain in the case that they are not genuine MLP ribbons and that you received only 18 rather than 20 too. Also explain that you already contacted your seller to try and work things out
I think if I were you then I would be wanting to do a return for a full refund (sellers cant refuse to accept your return since ebay can make them if the items you received were not as described.)
Hope that helps a bit
Please let us know how it goes