One thing that works in my favor is that unless you go out of state, it's pretty hard to find a low mileage, nice condition AP1 (00-03). I happen to have an 02 with 63k miles so it's a highly desired car thanks (in part) to being low production. I guess you could say if they don't like it, they're most likely SOL.
I buy all of my parts at a significant discount. For example, the KW V3 coilovers I have retail for $2300 new, I bought mine for $1650 new, if I decide to sell them I'll get around $1800. I paid $1k for my hardtop, I'll sell it for close to $2k, I paid $650 for my Recaros, I sold them for $1k + stock seats (about $1500 in value), etc. I NEVER buy a part unless I know I can resell it for what I paid for it or more. I also
Hell, a few months ago I bought a bunch of audio stuff for $200, got a nice head unit, an S2000 specific adapter, two component speakers, an amp, and some other stuff. Turned around and sold the head unit + adapter for $190, got the amp + speakers for $10 essentially. I can sell those right now for about $250 total.
Essentially the key is to avoid depreciation. Why be a dumbass and buy an $800 exhaust brand new, take the instant hit in depreciation and then only be able to resell it for $450 when you can just buy it used for $450?