Both sentences are perfect. Either can be used. Here's the difference: The sentences refer to three instances in time: (1) the present moment when the person says "you told me...". (2) the past moment when "you told me" (3) the moment furthest in the past when the item was purchased. The first statement uses (2) as its reference frame. At that instant, (3) was even further in the past so it warrants the past perfect "had bought". However, the second second sentence is also perfect. It uses (1) as its time frame. From the perspective of that moment, (2) and (3) are both in the past so both can be spoken of using the simple past: "you told me" refers to (2) "you bought it" refers to (3).

Both are correct, although the first one has a stronger emphasis on the fact that the action is complete.

In the U.S. either one works. Second one more common in an informal setting. Only thing I would personally change is the word "had" to "have". So it would be "You told me you have bought it", but that's only a tiny nitpick.

It s backsfitning in reported speech and it depends on an original direct speech statement. For the past simple there is usually the past perfect. So the expression with 'Had bought' is correct. For more information about reported speech please check the backshift table rules.

In the U.S., both are grammatically correct. The sentences, however, don’t mean exactly the same thing. The use of “had” suggests something happened after “you bought it.”