We all share personal tales of border-crossings. Whether it involves coming back to the US or passing through several Jordanian check-posts near the Golan Heights, border-crossing is subject to rules and interdictions. Personal effects can be searched at the border. Even if you cross a border without being checked, you are still spontaneously supposed to respect the border regulations. Modern nations impose rules on the movement of persons and goods. In fact, whole arrays of modern regulations characterize modern border-crossing, such as banning the import of meat or a gallon of olive oil. In some cases a visa of entry can be denied. In other cases, the required conditions in order to obtain a visa are such that you might just prefer to stay home instead.
Ancient Greek cities had their own rules. In general, people were free to travel during peacetime, but different controls were enforced. At some time, it seems that the Athenians needed a leave of passage when crossing Boeotia on their way to Delphi (Aristophanes, Birds, 187-193). A paragôgion was perceived by the Kondaians at their border with Gyrton (or Gonnoi), in Thessaly. Things could get nastier during conflicts. At the outbreak of the Peloponnesian War, the Athenian voted a decree to ban Megarian goods from the Attic market and the harbors of the Empire ( Thucydides, I 139.2); at one point, Athenian citizens were expelled from Sparta (Thucydides I 144, 1). Products were generally taxed when entering and leaving the harbors of a city-state. In Athens, foreign imports were subjected to tax, and merchandises were inspected by customs officers at the Piraeus. These regulations gave way to smuggling: the most common procedure was to unload the ships‘merchandise in a remote part of the territory in order the avoid paying the 2% tax, a practice denounced in Demosthenes’ Against Lacritos (35.28). Proxeny decrees often include the ateleia, which is a tax-free privilege accorded to the beneficiaries. In other cases, cities could establish free-trade agreements. In fact we know of a lot of regulations concerning the imports and exports of goods at different periods. 
But what is more difficult to understand is if similar regulations were also applied to persons and goods crossing the land border on foot. The question has not been systematically studied. According to Xenophon, Attica received many goods by land (Poroi, I, 7), and I see no reason to doubt this. Sure, the volume of land imports was considerably smaller than the one from maritime imports. Due to the cost of land transportation, it made probably no sense for Boeotian merchants to sell large quantities of foodstuff to Athens through the Parnes’ bumpy roads. But small quantities and specific products could surely be put on pack animals, transported to Athens and sold in the Agora or elsewhere in Attica. Aristophanes gives us a list of goods that peace would allow to flow back to the marketplace: from Megara, garlic, early cucumbers, apples, pomegranates and little cloaks for the slaves; from Boeotia, geese, ducks, pigeons, larks, and baskets of eels from Lake Copais (Peace, 1000-1005). Foreign goods were imported to Attica through land routes, and it is reasonable to believe that these imports were taxed. The remaining question would then be to know how this was effectively achieved.
It is probable that custom officers controlled the main “ports of entries” inside Attica: these would then be located on the coastal road west of Eleusis, at the Doskouri saddle on mount Kerata, at the saddle entering the Koundounra valley from the west, on the main road near Oinoe, and by Dekeleia and Aphidna. Some modest custom-houses were probably built as well. But who enforced theses controls? Astynomoi, agoranomoi and “port officers” are attested for the town and the Piraeus (Aristotle, Constitution of Athens, 50-51), but none for the rest of the chora. In the demes, the demarchoi were responsible for maintaining order and accomplishing diverse tasks, including in the area of finance . However, it is doubtful that the demarchoi of the border demes disposed of enough time to control and tax imports. Instead, this might have been the job of some appointed tamiai. Such regulations might have led to the emergence of an activity typical of borderlands: smuggling. Avoiding custom officers by using the numerous paths over mount Parnes was certainly an issue, but it was risky. The borders of Attica were well-guarded, not by a “network of fortifications,” but by mobile patrols and young recruits. And even after entering the borders, some sort of control must have been exerted at the market places themselves. So a Boeotian walking to Athens in order to sell his basket of eels would be better off declaring it at the border, pay a tax in exchange of a token, and safely sell his merchandise in the agora. Unfortunately, this reconstruction of events is purely speculative.
 G. Lucas, Les cités antiques de la haute vallée du Titarèse, CMO 27  88-89.
 See A. Bresson, L’économie de la Grèce des cités (fin Vie-Ier siècle a.C.) II. Les espaces de l’échange (2008) 72-101.
 On demarchs and other officials, see D. Whitehead, The Demes of Attica 508/7-ca. 250 B.C. A Political and Social Study (1986) 121-148.