I have noticed this phenomenon from time to time (though I don't spend a lot of time scouring Ebay). In particular, I've noticed "waves" (tsunamis?) of postings typically from India for their outrageously crappy instruments. To a lesser degree, postings for "direct purchase" sales from China. I'm not sure what triggers this, though in general you'll see increased postings for the Christmas season. Otherwise, I think the hypothesis about the return to school period may well be correct. The Indian postings attract a lot of ridicule (particularly on the tuba and trombone lists) because they often involve real howlers concerning the language (vocabulary and phrasing) used, and hilarious pictures of people wearing white gloves with the instruments and frequently holding them in awkward or totally incorrect positions.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)