I like threads like these because I can share my views and experiences, and hopefully provide some help too!
1) Yes to masking/painters tape on cases/toploaders/bags with cards inside. No to scotch tape and "card savers". Scotch tape will not only ruin the toploader, but can also cause problems if the card accidentally gets stuck to it. Card savers, as Kirk mentioned, really don't do much. I'm guilty of using both scotch tape and card savers in the past (once I even scotch taped a card saver closed - yikes!) and have since learned from my mistakes.
2) Trade size varies by member. I don't think I've ever completed a trade for more than 20 cards on here, although I have sent out some hefty care packages to some bloggers. Generally, it's because my FS/T list (which I still need to get back up on here) doesn't land a ton of matches to other member's wantlists. But you will find members who don't mind throw a single card in a PWE, throwing a stamp on it, and getting one in return. Most buying sites are going to charge you a minimum of $0.70 with shipping for a single card (thinking SportsLots) so trading 1-for-1 on here makes more sense.
3) Bubble mailer vs PWE: ask. I can't stress that enough. Ask your trade partner if they mind you sending in a PWE. I know shipping for a bubble mailer is expensive, so for just a few cards, many won't mind if you use a PWE. Just make sure it is packaged properly and the shipping amount is correct. I can't count the number of times I've had postage due on a PWE because someone thought it was okay to stuff 10 cards in an envelope and throw one stamp on it. I always take my packages to the post office. More time consuming and expensive (in relation to using PayPal postage)? Yes. But the peace of mind is worth it. For larger trades, I've found that a team bag of cards nestled and taped between two pieces of cardboard and placed in a bubble mailer is very effective. $3.50 will get it to where it's going in three days with tracking. But again, if it's smaller, ask your trading partner if they have a preference.
4) This is one you didn't specifically ask about, but I want to throw it out there. Do not be afraid to ship to Canada! I'm not from there myself, but many members are afraid of the high international shipping rates. In actuality, I was able to send member sandyrusty a handful of cards for just $1.70, properly packaged. He would be a good member to ask on the proper way to load up a package for Canada.
5) As I mentioned on another post, the fact that you understand why someone may ask you to ship first and that you are willing to goes a long way. Once you build up a reputation here, you will find that there is a great online trading community right under your nose (as well as a ton of other awesome tools!). If there are any other questions you have, ask away; that's what the forums are here for!
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.