Hi @srd
I believe that you will need to change the equality operator to == like so:
<?php if (get_field('transaction_type') == 'buy') { ?>
<td><?php the_field('purchase_price'); ?></td>
<?php } elseif (get_field('transaction_type') == 'sell') { ?>
<td><?php the_field('sell_price'); ?></td>
<?php } ?>