Texas is a non-disclosure state, meaning that sales information is not public information. There are, however, sources the appraisal district has access to that track sales information. In general, the appraisal district has more access to residential sales information than commercial sales information. This is because of MLS for residential properties. There are database services that track commercial real estate transactions, but they are very limited in the number of sales they are able to actually verify.
Sources the Appraisal District Uses to know (or guess) what you paid.
MLS for residential properties. This is the most common source the appraisal district uses to know what you paid for a property.
Public surveys. When a property sells, the appraisal district will send a survey, asking the property owner what they paid for the property. You are not required to respond to this survey, but some members of the public choose to volunteer this information.
Deeds. Mortgage amounts are published on deeds. The appraisal district will make assumptions about a loan-to-value ratio (LTV) to back into a sale price for the property.
Other proprietary databases such as CoStar, Xceligent, Loopnet, and others.
These are just some of the more common methods the appraisal district uses to obtain sales information about properties.