If you need crystals, learn how to edit them with CheatEngine. Other then getting the challenges theres no reason to actually use the weapons from the vendor as they are most likely weaker than what you have.
And as ive stated before, the items are the SAME each time the shop resets. what you see is what you get, minor variations in values thats all.
Plus chances are Cal doesnt have the DLC, and if he doesnt i doubt hed spend 10 bucks just for ONE option to add to the trainer.
Heres How for those that do not know how.
Install latest version of CheatEngine.
Click the flashing PC icon at top left of CE and associate CE with BL2 selecting Open on the borderlands2.exe.
When you are able to pick up a few of the crystals, pick up 1, tab to CE, enter 1 in Value and hit First Scan. Pick up another Crystal, goto CE and enter 2 into Value and hit Next Scan.
Do this a few times until only a couple addresses show up.
Double click one of the addresses to add it to the list, double click the value portion and enter 99 and hit ok.
Go back to the game and check the amount of crystals you have. if it was the right address it should say 99, if it doesnt try another address.
When you get the correct address, go back to CE, change the value to 120 as thats the max amount that items cost, to the left of the address click the little box to make the value constant.
There you go, problem solved.
Or as mentioned before just.. use.. the.. save.. editor.. that someone posted the link to.
If Cal and x90 dont have the DLC, theres no point in them paying 10 bucks just for one option.
Theres also save editors posted on these threads that give crystals as well.