I also reside in Netherlands and I have used:
- Markplaats because it would give me an option to pick up cards in person. But very few times since I think the prices are always a lot higher than market. All of the good deals are already gone.
- Catawiki for auctions, they are reliable and have money back feature on non delivery.
- Best would be ebay and cardmarket.eu since you have all of the safety and guarantee required as a new collector.
- There is also this thread which was created to list down the Pokemon convention events in Europe and there are always plenty of Pokemon events happening in Netherlands. I personally feel it is great to go to an event if you have the time and have a look around. I feel it is a small but a great community and showcases a lot of vendors and services. A lot of good deals can be made just by getting your cards in binders. You get a feel of the card prices and what is popular and what is not. Most of the event at these venues happen almost every quarter or atleast at a minimum 2 time a year (summer and winter edition).
Regarding strategy I think everyone has already shared the best tips. I am not a seller myself but I think @pfm advice is very much spot on. The market is already highly optimized and if you are on a budget then the best option is to buy what you want to, within that budget. However, if you want to open some packs also involving your kid and get a feel of the hobby and the joy of opening packs, you cannot go wrong with the modern sets.
Have fun!
Cheers!