Hello. Contrarily to what happens in english, I would spontaneously say: "Lo único que compro son + substantive (pl)". For example: "Lo único que compro son manzanas". In your sentence, I would prefer to say: "Lo único que compro es fruta". AS A GENERAL RULE, in spanish, in copulative sentences (subject + copulative verb [ser, estar, parecer] + predicate), when: - subject and predicate are substantives and one is in plural and the other in singular, the verb is in plural.

If you choose the definite noun phrase "LAS frutas" (instead of "frutas", "fruta", or “la fruta”, which would also be correct), the verb must be "son", not "*es", cf. "La única cosa/Lo único que compro SON/*es LAS frutas"; if you choose the indefinite plural count noun “frutas”, SER must again appear in the form “son”, not “*es”, cf. “Lo único/La única cosa que compro SON/*es FRUTAS.” Only if you choose the collective singular mass noun "(la) fruta", in definite (+la) or indefinite form, must the verb “ser” appear as “es”, not “*son”, cf."Lo único/La única cosa que compro ES/*son LA FRUTA"; "Lo único/La única cosa que compro ES/*son FRUTA". Those are mere facts, though; the question is WHY? If you want to know, here is a little exercise for you: Facts: F1) when surrounded by two noun phrases, “ser” can express IDENTITY (A = B) or CLASS-INCLUSION (A belongs to class B); F2) when it expresses identity, the two NPs that flank it are ‘fully reversible’ (by definition, [A = B] = [B = A]) and EITHER can act as subject; F3) When it expresses class-inclusion, on the contrary, the NPs are NOT ‘fully reversible’ (by definition, [A belongs to class B] is NOT equivalent to [B belongs to class A]) and only ONE can act as subject; the other must be the ‘attribute’, even if it can appear before “ser” and SEEMS reversible; F4) subjects may FOLLOW their verbs, cf. “Juan llamó” – “Llamó Juan”, each appropriate in a different context. Questions: Q1) In which of the four sentences above does “ser” express identity/class-inclusion? Q2) In which are the two NPs ‘fully reversible’ and in which are they only apparently reversible? Q3) Which have the NP “(la/-s) fruta (/-s)” and which the NP “lo único/la única cosa que compro” as SUBJECT? I do not offer you the explanation, only the way to find it, because the ‘gurus’ here would get very irritated if I did! :)

- Usually, as you say verb has to agree with subject : Yo soy carpintero - but if object is a personal pronoun (yo, tú, nosotros,...), the verb must agree with the object (El responsable soy yo) - and in your case, when object ans subject are nouns or similar, and one of them is singular while the other one is plural, then the verb usually agree with plural noun: (eso son falacias, lo único que compro son las frutas) Source: http://buscon.rae.es/dpdI/SrvltConsulta?lema=ser (2.1.1)

Se dice: "La única cosa que compro ES fruta" La palabra fruta es contable e incontable, dependiendo el contexto. me gusta la futa (en general todas) me gustan las frutas como la manzana, la pera y el durazno (es más especifico)

ES. La unica cosa que compro es fruta (Fruta, cosa... singular so verb too).