True story from one of my former TEFL pupils in Poland: She grew up in a small village near Krosno on the border with what was then Czechoslovakia. In the early 80s Poland was bankrupt and various items* were rationed or simply not available but you could still cross the border (other than for about 12 months during martial law) and no visa was required to enter Czechoslovakia. Nevertheless, border crossings were time consuming and bureaucratically demanding** and frequent crossings returning with Czechoslovak goods would be liable to attract official attention. But if you were a little girl on a bike, you were just waved through no questions asked and could return with a basketful of all the things your mum had asked you to get! I presume Czechoslovak banks would exchange Polish currency.
*Meat, confectionery, tobacco, alcohol, clothing were only on ration cards. Furniture was only distributed to newly weds and citrus fruit was imported (from Cuba) once a year, in time for Christmas. Toilet paper was only available in exchange for old paper, 1kg got you one roll. Bread, matches and for some reason vinegar were the only items reliably available in shops. Inhabitants of rural areas did not get meat vouchers as it was assumed they kept their own animals.
**You didn't get a passport to keep, you had to apply for it in advance of each foreign visit, with proof of planned trip etc, and return it when you reentered the country. While you were abroad, your national ID card, needed for various other bureaucratic procedures, was held in the passport office to be returned when you handed back your passport.