Can you be arrested for trying to by drugs?

Answer 1

If you are suspected to have sold drugs by a federal or state police authority, you can be detained, questioned, and/or have your property or person searched (with attainment of a warrant) with the allowances of probable cause (or in the final case, the provision of a warrant by the proof of probable cause).

Probable cause, in turn, a "reasonable belief that a person has committed a crime." As broad as this definition is, so too has been its application and attainment in criminal law.

Answer 2

It depends on your state/country laws, but most places have laws to prevent "loitering with the intent to obtain controlled substances"

Answer 3

Yes