What type of pearls are these and where can I sell them?

That is a graduated vintage akoya strand made by Mikimoto. These strands sell well on eBay.
Do a search to see what strands with that clasp (or another silver clasp, with the Mikimoto mark) sold for recently. That should give you an idea of how to price it.

The only problem selling them on eBay is that you need to have sold before on eBay and have gotten feedback & most Mikimotos have to go thru authentication. The strand can fail, if the x-ray machine can't read the metal or it isn't 925 sterling, if you said it was. (That happened to someone on this forum.) So maybe look around your location & see if you can sell them there at a consignment jeweler. Check eBay solds for comps. Some towns pearls will sell for more than eBay. If you can sell on eBay, just get the metal tested with x-ray from a local jeweler so you know what it is and that it will pass x-ray.

My understanding is that the seller is only obliged to offer authentication service for jewelry if the price is $500 or more.

With the name Mikimoto that seems to make it vary a little... there is a chat feature that you could ask eBay those questions.

The authenticity guarantee is optional under $500, to be added at checkout; that is what I see on current and completed listings.