App searches exchanges for the most favourable pricing source.
The taxpayer has to be consistent with how they determine the price of bitcoins, generally meaning that they have to use the same method unless they have a good reason for switching. Deliberately choosing the most favorable pricing source is fine the first year--but they'd have to stick with the same source in subsequent years.
Also, the app would be enabling tax fraud. A "prominent disclaimer" isn't going to be much protection. If anything, it's likely to be used against the appmaker to demonstrate willful blindness.
A taxpayer has to be internally consistent in picking their pricing source. It does not follow that the pricing source must be consistent between tax payers. Different situations could merit different pricing sources. It is not wrong to advise different people in different situations on how their differing needs may merit different pricing sources.
The taxpayer has to be consistent with how they determine the price of bitcoins, generally meaning that they have to use the same method unless they have a good reason for switching. Deliberately choosing the most favorable pricing source is fine the first year--but they'd have to stick with the same source in subsequent years.
Also, the app would be enabling tax fraud. A "prominent disclaimer" isn't going to be much protection. If anything, it's likely to be used against the appmaker to demonstrate willful blindness.