The Technics theme doesn't perform very well, but if you're a fan then you can probably make enough to take the edge off of an expensive hobby.
Sets are hit or miss. For example, check out the performance of 41999 vs 9398. The first was a limited edition version of the 2nd, but you wouldn't necessarily know it by the current prices. The old Ferrari sets did well, but the collecting game has changed a lot since then and I don't expect the Porsche or Bugatti sets to perform similarly (longer production time + more hoarding = lower prices down the road). Construction vehicles have been steady sellers, but buy-in is a key point to winning---so be cautious of any hype. The Bucket Excavator 42055 was/is amazing, and well received when it came out. But discounts were rare and now, 6 mos after retirement, it can still be found close to MSRP. By contrast, the 42053 Excavator was commonly available at 20-30% off (and very briefly at 50% off) here in the US and is now selling for ~2x MSRP.
Parts do a bit better, but it takes more work. Motors and power functions parts are steady sellers, and the common black friction pins are a top selling part on bricklink. Technic parts aren't in high demand in general, though, which makes them easier to pick up at a discount. One strategy for sourcing parts is to keep your eye open for sets from other themes that are on clearance. Chima and Ultra Agents were both great for this---buy at clearance, sell off the minifigs and system bricks, and keep the technic beams+gears+etc for building. A few Lego Movie 2 sets like MetalBeard's Trike 70834 is a current example---or will be in 9-12 months when it goes on clearance. Another place to watch is 2nd-hand stores, like goodwill or savers here in the US. I picked up a technic car last week at 1/2 off the price of new. A decent deal by itself, but it also held a power functions motor and battery pack, which will bump up the resale value considerably when I go to resell it.
Good luck, and happy building!