I recently stumbled upon several sites on and off Facebook for used items in Budapest that may be useful to those looking to buy or sell something. While I have not yet purchased or sold anything on these sites, they are quite active and could be useful:

Mom2Mom is run by members of the local Budapest Mom's group. This English speaking group mostly for expat moms or Hungarian moms married to expats features baby and mother toys, clothes, shoes, accessories, furniture. Budapest Moms also hosts a Mom2Mom live sale once every few months at the Benczur Hotel on Benczur u.. The next live sale is on September 23.

Used Stuff in Budapest features everything from electronic goods, to clothes to flat rentals.

Used Clothing in Budapest. I recently joined this group, but it seems to target women's clothing. 

In the US, I used to love spending Sundays going around to yard sales. There is also a site on Facebook anouncing garage sales and yardsales.

Expat portals such as  Xpatloop.com and  Caboodle have a market or classified section. Moving company Move One hosts Erika's Attic for classified listings.

A customer recently pointed out the free stuff site in Vienna, which could be worth someone's while, but as I do not speak German I cannot review it.

I used to host swaps in my home many years ago, and they were always a hoot. Anything you had but did not want anymore that someone else may find useful can be brought to the swap location. Clothes are put with clothes, jewelry with jewelry, toys with toys, books with books, electronics with electronics, etc. Dump your stuff off and take whatever you like from any section. Any unclaimed large items such as furniture must be taken home by the person that brought it. Everything left over was taken to charity like the Red Cross or Salvation Army. It is fun watching everyone try on each other's clothes, especially if some alcohol is involved.

If you like Ebay, there is a Hungarian version called Vatera, but just in Hungarian.

There has been a lot in the news lately about fake apartment finding agencies, that most ads in Expressz and other newspapers are also false and just connect you to agents, etc. Maybe try these two facebook groups, though i have not tried them: Helloroom and RoomMatesBudapest.

There are of course flea markets at Ecseri (XIX Nagykőrösi út 156, Mon-Fri 8am-4pm, 6am-3pm Sat, 8am-1pm Sun) and outside at Petofi Csarnok (M-F 09-17, Sat-Sun 8-14).