I'll answer your questions in order, based on my experience and to the best of my ability.
1. Selling at an event vs online - It depends. At an event, let's say you meet someone who is in desperate need of some Lysandre. He didn't run any in his deck for whatever reason, and after seeing other's decklists, he decides it might be smart to add a couple copies. Let's say you have two spares. Normally, you can sell Lysandre for $1.85 on Ebay, but since he needs these cards now, he is probably willing to pay some extra cash for them. So instead of $3.70 for both copies, you might be able to sell them to him for $5-6. Not really moral, but it happens, and it very common, so don't feel bad it you do this. So if by better you mean making more money, then in certain situations, events can make you more money for cards.
2. Price difference - Online sites such as Ebay are generally the most accurate representation of price at the moment. People might be willing to pay more at an event because of short notice and convenience though.
3. Best store to sell to - Okay, here is my personal opinion. STAY AWAY FROM TROLL AND TOAD! I could (and have) go on and on about how bad Trollandtoad experiences have been for me, but I'll save you the time. I personally use Ebay for selling my cards and buying them. I find they are the easiest to use and you can generally sell cards on Ebay for higher prices than what TrollandToad will pay you. However, Ebay is tricky when you are the seller, as they tend to favor the buyer in almost every situation. Make sure when you say a card is mint, it is MINT (as in after you pulled it out of the pack, you put it straight into a sleeve, haven't touched it since. Some people are very picky, and if they aren't satisfied, you are usually looking at a full refund and having to have your cards shipped back to you. I haven't personally tried TopCut, so I have no idea, and Collector's Cache I find to be over priced with some cards that I know I can get on Ebay cheaper. (Example: Ebay is selling Yveltal EX promo for $6.75, Reg Art for $9.99, and Full Art for $24.99; while the Collector's Cache is selling Yveltal promo for $8.99, Reg Art for $11.99, and Full Art for $29.99)
4. Selling older cards - Personally, I would sell the cards to fuel my current expense with this game. If you choose to sell, keep reading this, if not, don't bother. Any older cards that you KNOW are worth a decent amount ($5-6+) I would sell individually online exclusively. Hardly any card shops will pay you anything more than $1 for cards out of rotation, simply because they are harder for them to sell because they cannot be used by the players ( a large majority of their customers). Online, would be your best bet to sell individual old cards with some worth. Collectors are always looking for old cards to either complete their sets or just to relive the nostalgia and build rotated decks. As for older cards without any real value, just sell lots on Ebay. Those seem to be all the rage now. Selling lots of 10-20 cards for $10. Just look up "Pokemon Card Lots" to get a general idea of how they work, and find a hot listing that is selling a lot to find out any tricks to selling them (it's beyond me how they do it).