Translation:This is the book that I've been looking for.
But it is a nice phrase.
There are multiple web sites that debunk this myth pretty thoroughly, a simple Google search with that exact phrase will yield at least a few. The gist of it is that in English language, a lot of prepositions are not used as prepositions in certain scenarios.
I will quote only one of the webpages, showing examples of perfectly acceptable sentences that end in a preposition:
- But there are many sentences where the final preposition is part of a phrasal verb or is necessary to keep from making stuffy, stilted sentences: “I'm going to throw up,” “Let's kiss and make up,” and “What are you waiting for” are just a few examples.
That sounds improbable to me. Maybe you're right and they constantly make grammatical mistakes (unlikely) or they speak a regional language variant in which that is acceptable (I wasn't able to find anything on that).
Are you sure you are not confusing the gerundio (buscando) with the participio (buscado)?
It is possible to make constructions like: "el libro ha sido buscado" which means "the book has been sought", or "la muchacha ha sido castigada" which means: "the girl has been punished".
Saying sido/estado would change the meaning of "have been." I think the same types of rules apply to using ser vs. estar. Since estar is used with gerunds (estoy caminando) I think that's why they use estar here. But if you wanted to say "I have been tall all of my life" you would use sido (ser) because I'm describing an innate quality. ??