I think it will be like this
From
<input type="text" size="10" name="condition" class="do_input"
value="<?php echo get_post_meta($pid, 'condition', true); ?>" />
To
Suppose the value will be "Other"
<select>
<option value="<?php echo get_post_meta($pid, 'condition', true); ?>" selected="selected"><?php echo get_post_meta($pid, 'condition', true); ?></option>
<option value="New">New</option>
<option value="Ued">Used</option>
</select>
You need to put logic in the "selected" attribute whether if it's selected or not by using tertoary operator.