Can I sue in small claims court if a merchant refuses to sell to me.

I'm not aware of any statute which prohibits a merchant from requiring a minimum purchase on a debit or credit card.  Such a statute seems unfair to me, since merchants have to pay a percentage of each credit card or debit card sale to the card issuer, however I'll take your word for it that Texas and California have such laws.  If you believe the minimum purchase was really a pretext, and the merchant was really discriminating against you due to your race, religion, etc. (i.e. if the merchant allows people of a different race to make 99 cent purchases with a debit card) as addressed in the Civil Rights Act, that would be a different story.  As far as your potential law suit, how were you damaged, that is how did you lose money due to the merchant's conduct which you think was improper?  Absent such monetary damages, you cannot sue in most small claims courts, because most are courts of limited jurisdiction and can only grant money damages.  A court of general jurisdiction may be able to give you equitable relief, i.e. enjoining the merchant from requiring minimum purchases before honoring debit cards.  Bottom line, although I don't think you would win, your suit may not be legally frivolous in the sense that it lacks any legal merit (after all, if the statutes to which you refer provide as you say you do, some legislatures apparently agree with you that the merchant did something wrong), but some (I am one) consider it frivolous to waste a Court's time with such minor grievances.