Isn't the whole point of collecting video games to hoard them to some extent? I've certainly never sold one of my games before

For some people perhaps, but that's not the plan for me. I collect because I enjoy playing games. I collect the games not the cart/disc, though to an extent I like my games to be presentable; I want a box, manual, (if it has one), and cart/disc all in good condition and with all the relevent information intact. Digital distribution just doesn't seem good enough to me because it doesn't feel like I genuinely own it. That said, I don't want to keep my games just to look at them.

I think it's different from other collections. If you collect coins, stamps, geodes and crystals... anything really... you're likely collecting something that has no real primary purpose, or a purpose that's irrelevent ie. the stamps are for posting things, but you're not sending letters. Game collectors who don't play games, well, they're not gamers, and not really game collectors, they're cart/disc collectors. I am a gamer, I play first, collect second, but I like to maintain a collection because I can enjoy the games the way I like them. If I'm not going to play a game though, it's simply wasting space in my collection that can be used for something else I will play and space is finite.

Honestly, if a game collector says they've never sold part of their collection, they've either lying, have not been collecting very long, or they're a damn millionaire. Houses have limited space after all, and every serious collector I know has sold off huge chunks of their collection because they want to focus on a single system, or streamline their collection, and they simply have no more space in their house for more games. After all, if we consider just 16-bit, if you collect a full Mega Drive and SNES library, you're looking at over 700 games EACH; without a mansion or a second house specifically to store it in, where would you keep it all?

OP:
I almost always follow your rule #3, that BTW almost always satisfies rule #4 too. Rule #5 not an issue for me, my backlog is huge due to lack of time, but sooner or later I want to play all my games. I don't follow rule #2 because, as I prefer physical format, if I find a game I'm interested in at some shop at an attractive price, I get it, as delaying the purchase and possibly not finding it anymore, could cost me more time and money, either having to order it and pay for shipping, or ordering it at a shop and not obtaining anymore a time-limited discount. I tend to buy few games anyway.
I'm currently a PC gamer, but I'll most probably follow rule #1 if and when I'll buy a console again (my last one was an Intellivision).