Labeda, sorry for that.
If you still have copies of your agreement (email, chat, etc.) and considering he paid the down payment (you must have a proof too), then the agreement you had is valid and still stand. You send reminders but got no replies, the first thing I would do then is to send an invoice for the remaining posters with interest - the cover note should state terms of the agreement, date and posters and highlight the lack of replies. Be nice, give him an alternative, that is to return the posters within seven days with proof of shipping. The amount you are asking is below the small claim court (most small claim court in the US starts from 2500 USD); so if you are have a lawyer, resend the invoice with interest and letter of demand from lawyer a month later. And continue to do so, every month, with interest; once you reach 2500USD, action may be taken. All this to say that action is required from your side to have proof and valid case, just invoice, again and again; demand letter, etc. Do the layman lawyer first, be firm and always state the agreement terms and the fact you never received the posters nor the money; then your invoicing and demand letter is totally valid, not sure about the interest on debt.
I also got ripped off by a seller here who run a mess and failed to ship my goods... yet I have paid for them... I have tried to get my posters by sending numerous emails and the guy was a bit nasty, telling me it has been years (yeah sure, I send a few reminders over the years) and I got the goods (well, I have emails where he says he found them and will ship them ASAP). But he never did, I guess he resold them at his auctions. Just like you, I had to consider shaming, then his replies were just terrible, I got insults, telling me to go fuck myself, etc. The funniest of all, this guy is in this forum and went I raised the matter, the all discussion was that dealers should "
not to deal with terrible customers like me who don't track his purchase". He wanted me to pay shipping on the two items he found back and I refused... Ahahaha, this guy managed to turn the situation and he played the victim. Guess what, it worked, he got support here... even after insulting me. He even used my story for promotional purpose, saying that next buyer will get some of my stuff. He is a well loved person here, so I am the asshole in the story
Some people can be really arrogant and BS... and so cheap than a dollar is a dollar. And some people are really biased, even if this guy was right, the way he replied in this forum was just terrible. Seeing the good side of the story is helpful, while I am enjoying home-working from a tropical beach with a well-paid salary, this guy is presumably still arranging his trucks full of 1-dollar poster craps so that he can make ends meet. He can keep my dollars but he deserve to be reminded of below-to-zero attitude.