Morpho-Syntactic Decline of the Roman Empire

caveat emptor is in the Subjunctive Mood.

Not the Imperative.

Meaning not just buyer beware

but let the buyer beware

Subjunctive Mood doesn't denote just prohibitively that the root verb is not a command and therefore not Imperative Mood (that is, B is not defined as just ~A)

but denotes positively an emphasis on the potentiality of the execution of the root verb.

May/might/should/ought...whatever. If you're stuck in English you can also use the Indicative Mood of allow et al, and stack up infinitive verb forms until the person next to you who is not interested in grammar at all starts crying. Or choking you.

Thus.

caveat emptor

is not, was not you buyer beware, use discretion in your purchases, you must do this

caveat emptor is, was

buy that right now. buy two and put it on plastic...that's what credit cards are for.