Learn Icelandic: Maður getur fundið ýmislegt á flóamarkaðnum. - One can find various things at the flea market.

Ýmislegt is the neuter substantive form of ýmis (various/several). We use this specific neuter form when we aren't describing a specific noun, but rather talking about "various things" or "assorted stuff" in a general sense. If you were describing a specific plural noun, like "various books", you would use a standard form like ýmsar bækur.

Why is the verb fundið used instead of the infinitive finna?

The modal verb geta (can/to be able to) is a special case in Icelandic: it is always followed by the supine form of the main verb, never the infinitive. That is why we use the supine fundið instead of the infinitive finna.

Why does the preposition á trigger the dative case for flóamarkaðnum?

The preposition á takes the dative case when it describes a fixed, stationary location (being "at" or "on" the flea market). It would only take the accusative case if it were describing motion towards a destination (going "to" the market).

Is maður the normal way to say "one" (as in "one can find")?

Yes! While maður literally translates to "man" or "person", it is the standard, everyday way to express the impersonal "one" or a general "you" in Icelandic.