- Clarify and correct direction of trading:

- Do not exchange "costbuy" and "costsell".
--  Shop buys  : player sells : at costbuy  : with buttonsell
--  Shop sells : player buys  : at costsell : with buttonbuy
- Fix barter shop text display.

Signed-off-by: Xiong Changnian <xiong@cpan.org>