I thought it would be a good idea to make a thread where everybody can share their experiences about the various online shops and marketplace sellers.
So, for example, if somebody is not sure whether a marketplace seller's packaging would be adequate or not, just check this thread and maybe somebody has some info about it.

Please differentiate international and domestic shipping.

Here are mine (always international):

Bookdepository: I've never ordered from Amazon but based on what I've seen, Bookdepository has better packaging in 70-80% of the cases (amazon owns BD). Sometimes they put bigger tpb-s in bubblewrap envelopes instead of cardboard boxes, but they are pretty good with refunds and replacements and I've never had to send back anything, I could always keep the damaged items.
Just this week I got a replacement copy of a Showcase Presents book, because the original order was badly damaged in the crappy bubblewrap envelope. The replacement copy arrived in an even thinner bubblewrap envelope and it looked the same. I sent them some photos and they gave me a full refund and I kept both damaged books.
Today 2 Essentials arrived from them and both were in cardboard packaging, in mint condition. So it is a bit random.

Wordery: Similar to bookdepository, but their shipping is very slow (which is strange because they ship from the UK IIRC). I complained about a late book once and they immediately sent a replacement without question.

tfaw.com: Best packaging I've seen so far, never had to ask for refund or replacement but I have a friend who got a damaged figure and the immediately sent a new one. Another friend ordered a nick & dent hc 2-3 month ago and it arrived only in a nylon bag. He emailed them and they said sometimes they ship nick and dent books with this kind of packaging.

amazon.co.uk marketplace sellers:
Speedyhen: I think they don't ship to Hungary anymore through amazon but when they did they were the fastest and the cheapest. The packaging was very secure, never had any problems (always ordered omnibuses). Once I ordered 3 regular hc-s from them through ebay and they all came in cardboard envelopes. 2 of them had some minor dings but nothing to complain about.
I've heard that their packaging sucks a bit in case of domestic orders (UK).

bookoutlet2USA: the book was cheap, shipping was fast and it was in a cardboard envelope (which is OK for an Essential but not ideal.)
UK BOOKS DIRECT: same as the previous but the packaging was a bit better.
World of Books Ltd: same as the first
Books2anywhereUS the packaging was a bit weak but the book wasn't damaged (it was a hc, listed as and arrived in "very good" condition)

books_any A lot of people complain about their slow shipping but the one time I ordered from them the book arrived in 2 weeks (shipped from US) and it was in almost perfect condition (Sandman omnibus vol1, placed in a box with an airbag).

Bilbio.com marketplace:
The Book Place: I've asked them after I placed the order to check some info for me about the book (page count, content) and they've sent photos about the dustjacket so I can check it myself. They were very nice and responded quickly. The book (a hardcover) was listed as "good" but it was rather a "very good" despite the fact that it came in a bubblewrap envelope. Shipping took 2 weeks, which is good in case of US orders.

Bookshop4U Also from the US and it took 2 weeks. It came in a thin bublewrap envelope but it didn't look any worse than the listed condition (and it was dirt cheap).

[B]Abebooks:[/B]
Paperbackshop-US: Shipping always took 2 weeks (from the US obviously) and the books always came in a bubblewrap envelope with a piece of cardboard attached. Very minimal damage despite the week looking packaging (I ordered 3 big books from them and for the price, a microscopic little ding is nothing to complain about IMHO).

Sierra Nevada Books or Thrift books League of extraordinary gentlemen Black Dossier came without the 3D glasses (used book) and they immediately gave me a full refund and I kept the book. Costumer service was quick and nice.
The packaging was only a nylon bag but the book didn't look worse than the listed "good" condition. Shipping was slow but that was expected.

That's it for now, please feel free to share your experiences!