"Boot and Bonnet" English for what Americans call the "trunk' and "hood" of a car.
thus the expression a "Boot Sale" weekend sales out-of the trunk of a car.- or other days of the week.
In the early 90's I was working in Greece and there were weekly sales in different cities- Sunday it was Athens, Wednesday it was Salonica, Saturday Ionnina.
At this time there were many "Russian Bazaars" where one could buy dozens of Soviet cameras. I even knew of 1 local man in Salonica who would send a young boy ahead to spot the tables or car trunks full of cameras, lenses and boxes of those illusive 40,5mm filters light-meters, and even film paper and enlargers.
This man's Russian and Greek were better than mine ( as well as his bargaining ability) and he bought alot of the "choice" items since the sellers often knew little of what they were selling.
Other non photographic items included camping equipment, pots and pans, inflatable boats, scuba gear and even bootleg Soviet Elvis albums.
I heard that another great Fleamarket was in Krakow.