I've looked at Yahoo Auctions multiple times. Often, the price of the games themselves is very good.
However, by the time the forwarding company adds on shipping + other fees, the game ends up being $10 -$30 more than what I can find on Amazon, Ebay, etc.
Do people just buy in bulk? I see it recommended so many times but don't really understand why given the cost. To be fair, when I have looked, it is often for a single purchase.
Varies. Some of those places will offer discounts for bulk but it ultimately tends to end up proportional to amount of work necessary -- one trip to the game shop to buy 5 games being different to multiple things over multiple weeks.
A lot of what you will see on ebay and amazon from Japan is third rate Japanese, which might well wander into mint territory elsewhere (I know it is a bit of a meme but it really is true). Not so many Japanese types care to deal with Amazon and Ebay either (Japan kind of doing its own thing internet wise, indeed us talking about yahoo auctions being a demonstration of that) but I can't fault them for that one as there is a reason I don't deal with them either as a seller (or indeed buyer these days either really). That or relative/friend is there and can send things over -- hard to bargain over 1 game, easy to bargain over 5 and if 4 are going out the door then possibly even free game to me.
Some importers will be those that visit for whatever and take a trip to... would once have been Akihabara but today is probably somewhere else to liberate a few choice titles like you or I might cruise a bunch of game shops, charity shops, pawn shops and the like to get something interesting, at which point problem dodged as I don't think I have every heard of any such people being stopped by customs compared to booze and jewellery.
You also have play-asia, genki, yes asia and similar. These vary dramatically and do venture into older games as well - shmups, puzzle games non Japanese can read, speedrun fodder, decorative/coloured shells and region exclusives worth noting in this is not a hard list to stock up on, and indeed is the Akihabara run list of possibilities as well.
As above though some people do it for things they are not going to find otherwise, and damn the expense -- compared to most hobbies $10-30 is whether you have a burger or a better meal and buy a tshirt when you go to the event. There are also still some occasions where Japanese version is cheaper than some mega pumped thing in PAL and America (either because it is not big in Japan or sold well enough in Japan, see all those mega popular in Japan but unknown/comparatively niche outside it lists that usually start with dragon quest, for supply to be higher than outside it where it might have trickled out the back door with no marketing) and the western gaming press being all "I like the popular thing"*.
*this recent lark with Elden Ring seemingly catching them all off guard is far from new, for a slight twist to take us to older times I do recommend the alpha protocol comparisons between US and European reviewers.
