Can anyone tell me what bullseye is in Esperanto?

bullseye, if you disect the word into "bull's eye" is an english idiomatic expression for "directly hitting the target."  It probably stemmed from the fact that the eyes of bulls resemble targets used by archers, or perhaps gunman depending on when the term was first coined (i'm not exactly sure.)  If one translates it directly, you get "virbovokulo."  Unfortunately though, the idiomatic meaning does not translate.  If two english esperantist speakers were speaking together, and one used the word "virbovokulo" to mean "hitting the target" the other one may understand.  but if an english esperantist were speaking with a german esperantist, or a japanese esperantist, they would have no idea what you were saying. 

doing a search of "target" in la vortaro, i see that the esperanto word for "on target" is trafe.  so i suppose some sort of equivalent for "bullseye" would be "trafe."

If you mean the noun instead, maybe you want to use "pafcelo".