I don't think there is a
/select command.
/click only works on buttons and is to securely execute Lua onclick handlers.
If you're looking to select the editbox to type in a value, you need to call a Lua method to activate it.
Code:
/run TradePlayerInputMoneyFrameGold:SetFocus()
To have the macro set a value, you can use this. This example sends 1 gold. (10000 copper)
The editbox does not need to be "selected" in order for this to work.
Code:
/run MoneyInputFrame_SetCopper(TradePlayerInputMoneyFrame, 10000)