I was setting on that, when dwarven merchants came. I was like, cool, let's trade. I offered 9800 DB worth in goods, and selected about 9000 DB worth of merchandise. I clicked 't', but he said that he didn't like it, so I just rapidly pressed it, in hopes it'd work.[...]
The rest, aside, if they say "I cannot imagine us trading", or words to that effect, then there's practically no legitimate way you can get the stuff you requested, even if you were to select everything you'd currently managed to bring into the Trade Depot for trading. The only realistic thing you can do is to select less to 'purchase' while probably also adding to the stuff you're 'selling' in exchange for it.
Another reason why it's best to go for smaller lots. If you (frexample) had tried to attain 200 DB worth of goods with a 300 DB offer on your part, and you really couldn't have gotten away with less than 400 DB, you'd get a counter-offer (although usually for vastly more, so maybe 500-1000 DB, depending on various circumstances). If you're lucky, you might be able to remove some of the counter-offered items (or original ones) to lower it down to near the 400 DB acceptance level, or otherwise change the composition of the offer to items your opposite number seems to like more. But if you cut down your offer too much from their 'suggested improvement' then you're going to make them less willing to talk.
If you went for 9000 DB of stuff with 9800 DB (which is a very low percentage margin of profit for early on in the game, anyhow, so not surprised they wouldn't go for it), I'd imagine they'd be looking to make a counter-offer by including another 9000 DB-or-so of goods. Which you probably don't have. So all you do is annoy the opposite number, and eventually they say "Fair enough, maybe next year", i.e. the message that you got.
Also, when you get more experienced brokers (whatever skill it is again), you will get an indication about their mood that you don't have now. "...is willing to trade" is the usual starting point if you haven't annoyed or impressed them. "...is ecstatic about the trading" is (I think) what you might get if you've decided to be very generous (or especially provide some goods indicated from last year's liaison discussions as being things they're trading with at a 200%+ markup from base value). "...is starting to be annoyed" and "...is looking impatient" are something like the messages you don't want to see, and so if (once you have an experienced enough broker to even see them) they arise you're going to have to go for initial offers of double (or more) whatever you're asking for.
It'll give you a list of these on the Wiki, no doubt. Plus the special one for elves which means you've annoyed them by
what you've tried to trade to them, rather than just being stingy on the margins.
To keep them sweet, right at the beginning, a 10 DB unit of prepared slug eyeballs (or some other very basic foodstuff) might be best traded for a 30 DB better-than-base-quality stone earring. Although it's probably absolute total profit made, not individual percentages at every little small-scale transaction, that count so getting a couple of separate transactions getting 5 DB bit of base foodstuffs each in exchange for a 50 DB rock craft (each of 10:1 as far as margins are concerned) should not be better than a single transaction of 10 DB worth of food (either a single item or those two 5 DBs asked for in one transaction) against 100 DB of rock crafts (again, either a single 100 DB one, or two 50 DB ones).
However, I believe that there's at least anecdotal evidence that if you make two separate 5 DB <=> 50 DB transactions, coming out of the trading screen in-between, there's more increase in the broker's trading skill than if you put them together into a single 10 DB <=> 100 DB one. Probably not true, but I just zoom in and out of the depot like billy-o while I'm still trying to up the experience of the broker, anyway.
If nothing else, the broker skill could increase enough to make later transactions easier (with a potentially increasing skill, each time) rather than trying to do it all in one go and having to guess what margin is good enough for one huge transaction only at the level of skill that the trader arrived at the depot with.
Again, the Wiki will confirm (or deny, or at least give the best combined community guesses about) that particular mechanism, and you can't take what I've just said as authoritative without finding out what everyone else has said about it.
I'll stay away from the rest of your issues. (Save that annoying a caravan's probably mature soldiery with your trainee stuff wasn't a good idea, either; The issue of "tree, life, bones, cloth, thread" can also be answered by referencing the wiki, but I'm betting that the problem was either no bones or no silk cloth or thread; Don't worry too much about the blue stuff, some people keep away from it and never have a problem (picky nobles excepted), and others have FUN with it, but that's a discussion for another thread, I'm pretty sure.)