If you go on another one (you'll never buy/sell so much on only one merchant at level 1), you need to double this, since there will be 2 theft, so 126 books. And of course, all these books on the same merchant. You would have to buy 63 (!) books to start to compete against thievery (810/13). So basically, you have a discount of 13 gold on every book.
324 for every book (120 value in thievery mode). Most expansive stuff at this level are books. You have 4% discount at level 1 on bartering. The gold value is a special value only for thievery (something like x2,7 the usual values, so basically you can steal in fact something like 810 gold value). At level 1, you can steal for a value of 300 gold with thievery. With 1 point in thievery you can steal far more gold value than what you can spare with 1 point in bartering, and it's scaling all along the game.
:p Thievery is much better than bartering until very, very late in the game (and so much better in the end, since in the end you'll never need bartering if you had thievery before that).
