Egore108 - we were new last year, with a total attendance of 120, so our situation might be reasonably similar to yours. We had advance sales of the official CSTS merchandise (shirts, patches) through our local event website, and then ordered an extra 5 shirts (of which 2 sold at the event) and 20 patches (managed to sell 10 total). I had maybe 7 or 8 shirts from other sites (TeeFury, Ript etc.), some Jayne hats we knitted ourselves, and a painting that a friend made and donated. Apart from that, the only merchandise-type stuff we had came in the sponsor pack. We sold or successfully auctioned most things we had, but my feeling was that the supply:demand worked out fairly close - I think if we'd had more stuff, we would have been left with more.
Smaller items, like the keychains, would probably be good. I was surprised we didn't do better on patches. We also have to watch out for shipping and Customs costs, so ordering stuff from QMx etc. is a bit of a gamble for us; I won't do it unless it's on sale or I'm pretty sure it'll sell.
Another really important thing is getting the word out that there will be merchandise for sale, so that people bring cash. (Sorry, it sounds obvious, but despite us talking it up on the website, Facebook, Twitter, posting pictures of the awesome swag etc., a few attendees still said, "Oh, I like that, but I don't have money on me." We made sure to know the locations of nearby ATMs!)