"but for tax purposes their (cost) basis should remain unaltered."
You can elect (at least in the US) to use average basis (which is what you are talking about), FIFO (IIRC LIFO is not permitted), maximum capital gains or minimum capital gains. You can elect to minimize capital gains this year, but that comes at the expense of reducing the tax basis of the residual to 0
For stock, at least, you can exactly pick and choose which individual shares you are selling, UNLESS you are also buying at a similar time. That would be a "wash sale" and LIFO is Required in that case.
Comments
"but for tax purposes their (cost) basis should remain unaltered."
You can elect (at least in the US) to use average basis (which is what you are talking about), FIFO (IIRC LIFO is not permitted), maximum capital gains or minimum capital gains. You can elect to minimize capital gains this year, but that comes at the expense of reducing the tax basis of the residual to 0
For stock, at least, you can exactly pick and choose which individual shares you are selling, UNLESS you are also buying at a similar time. That would be a "wash sale" and LIFO is Required in that case.