I think the coin boost would be a great help to some players, especially those who are lower levels. That said, I think it would've been even better if it had been a sales boost.
Even after all these years playing I still sometimes find myself waiting for an item to sell out so I can pack away the counter.
If the boost would double the sales instead of coins only I could easily see myself using it from time to time even if it did cost a few gems.