Hang on tight, guys - this is gonna be a doozy:
That depends on what type of resources you're thinking about buying. Wood, for example, is easily grindable & affordable; if you don't care to grind for a few hours a day, then you can always trade with other users for some stacks. There's no excuse for them to overprice a material that you & others can easily obtain on your own, so you'll most likely run into sellers who're selling copious stacks for maybe a few thousand bells or a few Nook tickets.
If you're wanting rarer resources such as snowflakes or mushrooms, those can get pricy very quickly; from what I've seen, most people have chosen the Northern Hemisphere, so the demand for visiting a Southern island is high. I'm unsure about the forums here, but it's so, so hard to find anyone with a Southern Hemisphere in the New Horizons discord, much less have them accepting visitors for extended periods of time. Some do allow free entry, but others will probably charge you. That's where places like eBay come to play, but one major issue is Nintendo's ToS. Another big issue is sellers selling hacked-in items such as the wedding series or villagers (I don't know much about this issue). It's way too risky - you're placing your faith on sellers who might not be giving you an untouched item. The wedding series aren't even in the game yet, they were hacked in. Nintendo will most definitely find out about this and that could lead them to eBay or other sites. Sellers are more likely to get punished, but what about the buyers?
With that said, I've looked through eBay before; people love selling Raymond for a nasty price in-game, so it wasn't too shocking to find current bids & 'cheap' offers. It wasn't hard to find sellers selling off materials, DIYs, & furniture; most of those sellers actually had extremely high ratings & feedbacks. Some were even selling hard-to-get items for less than $10, some state they throw in extra gifts.
I honestly don't care, though - It's better to keep money away from this game, especially if all the materials needed are easy to trade/grind for. If you gotta spend some cash, it's better to do it for things that are hard to obtain or rare. Just make sure it's worth it though because if Nintendo comes through and bans accounts, including yours, then you can't be upset about the outcome.