A problem that we've encountered before, with no really good solution, is that the LEO team prefers to reserve the marking [rare] for really, really, really rare words, but it doesn't have any marking for words that are just very uncommon and that most native speakers would advise non-native speakers to avoid.
I agree that this word might fall in some category like that, but I'm not sure it's rare enough to deserve that marking. Someone might want to check a few more text corpuses before assuming that it doesn't exist in modern English at all.
In fact, I'm not sure it would jump out at me as wrong if someone used it in a business text about purchasing the wrong part or something. Mis-
is a prefix that could theoretically go with an awful lot of verbs, even if the results aren't all common enough to be listed separately in a dictionary.
The other problem is that Fehlkauf may be fairly common in German, just because German likes abstract nouns, but there are probably a dozen better ways to translate it than a hypothetical noun 'mispurchase.'
For the meaning of buying the wrong thing, buying something that shouldn't have been bought, you could say wrong purchase, mistaken purchase, purchasing error, purchased the wrong item,
A bad bargain, bad purchase, poor bargain, poor purchase, poor/bad decision to purchase,
etc. would be a somewhat different meaning -- buying something whose price is too high or quality is too low.
So I'm not sure if the German word actually means only one of those, or possibly both.
PS OT: related discussion: predominately