Lack of metal shouldn't impede your clothing industry (pig tails/collect silk), and in fact I you are short metal and have a surplus of labor... what else are they doing? Dirt is very quick to mine out to set up more farmland for pig tails.
The thing about cooked meals (roasts out of 4 ingredients), is that each ingredient gets a quality modifier for being prepared (masterfully MINCED plump helmets). Then, all 4 ingredients gets ANOTHER multiplier for combining them all. So each roast has 5 quality modifiers added to it (4 for the ingredients and one more ON TOP OF those for the meal as a whole). Thus, you can have a no-skill cook easily crank out tens of thousands of dwarf bucks worth of food for the first few caravans. Herbalism I very powerful with multi-hauling, and can easily supply most of that (especially with fruit trees).
Spiked wooden balls sell for a ton, but all trap components in general sell for too much. Wooden balls are just the easiest to make.
Both of these are usually considered on the exploity-side, though. The most popular alternative would probably be textiles for export. So masterwork GCS silk cloth masterfully dyed and masterfully sewn into a robe that has masterwork bone/leather decorations on it sell very nice.
As to your question, seizing once usually won't provoke a siege. However, setting up basic defensives (at the very least a way to seal your fort and turtle up) is one of the firt things that should be done. Eventually they'll get bored and leave (or you set up some traps to mince them down while they mill around your fort up top), so its no big deal.
The real problem is that a trader's inventory depends on the amount of profit the return with. Naturally, a caravan has a terrible profit if you steal all their crap... Next season, they won't have jack diddly squat to trade AND will be pissed at you (requiring you to pay more for less). Thus, if you were expecting the caravan to bring more metal items/food/cloth/etc next season, you will only be shooting yourself in the foot. Low quality, undecorated copper items can be bought for only a few hundred dorfbucks. You can probably pick up a few of those to melt down into more picks. The dwarven caravan is only one season away, so you can devote the next eason to building wealth and buy metal/cloth off them (using your new pick/s).
Also, if you just hate the clothing industry in general and never wish to fool with it, there is a trick to force caravans to bring a surplus of cloth/tread/leather. If a caravan arrives and you have less than 2 cloth items/clothing (spare) per dorf, they'll bring tons of the junk. So in the month before a caravan is due to arrive, go into your stocks screen an forbid all our thread/cloth. Once the caravan arrives, you can buy 10+ bins worth of the stuff before unforbiding all your stock. The same work for food and logs. Again though, if you seize all their goods this year, even doing this means they won't bring much next season. Particulary if you are on a metal-impoverished ste, you'll need to trade for metal (or kill for metal via sieges). However, you need good traps set up (repeating spikes, cave-ins ready to go, drowning chambers, etc) to kill off the siege without any metal of your own to equip your dorfs. Sending 30 poorly trained nekid wrestlers against armed goblins/humans usually doesn't end well. At the very least you need more wrestlers than invaders.
Basically, seizing is balancing short term profit over long term gain. If you just need that boost to get going its great, but if you actually need the caravans (wood in the badlands, metal when you have none) it can be counterproductive. If you feel seizing this load is near the last thing you need from the humans, then go ahead.