[quote name='GBM']So finally I have got material for two cables (1 original GBM linkcable, 2 original GBA-GC cables)

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Congrats! Where did you get the GBM linkcable from? They're extremely hard to find around here.
Personally, I would try to source secondhand Nyko cables instead of original GBA-GC cables, as the result will be a slimmer neater cable. The first time I looked on eBay I found one, but maybe I was just lucky that time.
[quote name='GBM']Is there anything I have to be extremely careful with while soldering? I'm not not that experienced....Is it possible to overheat/destroy the hardware easily? [/QUOTE]
I would say that soldering takes a bit of practice to get right. It's not just a case of heating up the metal & dunking a wire in it - there's an "art" to it, so that the wire heats to the right temperature, the solder flows smoothly, and so that you get a clean & strong joint when it cools.
Best thing to do is buy 5 meters of wire, cut some 10cm lengths off it and start practising soldering them together. Find some discarded old circuit boards & practise with them. I'd imagine there are "How To Solder" videos on YouTube and tutorials for noobs on on sites for electronics enthusiasts - watch some of those.
Don't start messing with your GBA cables until you already have your techniques right.
[quote name='GBM']PS: I'm planning to sell one of these on ebay. What do you think, how much will I get?[/QUOTE]
I don't want to sound negative, but if you've never soldered before, you probably shouldn't be selling the fruits of your labours. I can't paint, and I wouldn't set myself up in business as a painter - anyone would be able to see the mess I'd make of it! I think in your case you should be just glad you've got the spares as a contingency against messing up. You may find yourself glad of a second attempt!
Stroller.