I can help you a little I think.
It is "keinen" because "Käufer" is in its singular form, as mentioned on the upper commentaries, it can be "der Käufer - The buyer" or "die Käufer - The buyers".
So, in this case, it's "keinen' because "Käufer" is considered a male noun and also in this sentence it is in accusative form.
Also, you can check wheter it's nominative, accusative and so on, asking a simple question to yourself: "is the verb, that I'm intending to aknowledge, the subject of the sentence or not?" In this case, "Käufer", is not the subject, but WE. So when it's not the subject, must be declined.
Said that, then you'll have 3 genders to identify, akk, dat, gen.
Hope I could help!
@ Zivnew & voidlogic > ‘Käufer’ can either be ‘buyer’ or ‘buyers’ as in ‘Mädchen’ but in this case it has to be singular because of ‘keinen’ (Akkusativ masc./sing.). So this sentence would be “We have no buyer” which means what voidlogic has rightfully mentioned in English “We have no one to buy what we are selling”.
In English, you would either need to say 'We do not have a buyer' or 'we have no buyers' or 'we do not have any buyers'. Granted, there may be subtleties between these, but the message remains the same. So yes, singular works but you need to add 'a' before the noun to make it grammatically correct in English.
You could say "We have no buyer." in standard English, given the right context (native speaker). I could see a sentence like that coming up if you were speaking to people who had one individual item for sale - so there could only be one singular buyer. E.g. "To afford our new home, we really need to sell our old one. Unfortunately, since the financial crisis hit, the housing market is terrible. We have no buyer."