There are several major plot holes in S6, used to press home themes rather ham sisterly.
This is one of them.
S6 is, according to Joss, about dealing with life, so Buffy is handed the financial boogeyman on a platter. All the shoulda, coulda, wouldas would have gotten in the way, so we are asked to accept it.
Seeing as how we regularly buy into all the fantasy of the show, I guess I will manage to swallow this one...even though it's bugged me more than most issues on the show. It's a powerful example that dealing with real life can be even harder than facing the spirit world, and that Buddy's super powers are useless against it. One more way to remind us how alone she is, or at very least perceives herself to be.
Some of it is attributed to Buddy's moral high ground. She refuses financial help from Spike, because obviously it would be ill gotten gains, and it would have put her in a situation of obligation to him. She turns down Anya's suggestion she charge for her svcs on moral reasoning.
Interestingly she takes money from Giles...though his money essentially comes from being paid for doing a job that saves the lives of innocent people. I guess it's cleansed by being one level removed from the actual hands on slaying.
Police, firefighters and ems workers are paid for life saving...so maybe the moral issue only applies to mystical issues. We are consistently shown that there is a different sort of "economy" in the mystic world.
Watching the show week to week is a vastly different experience than binge watching and re watching. Sometimes things are presented in awkward ways or we are beaten over the head with themes because it's assumed that at least a week has gone by since we've seen the characters, that we may have missed an episode or two and need to be reminded etc.
I try to keep that in mind. We have the entire series set in front of us. Most of us have watched it umpteen times. I know those characters better than I know most people I see everyday. When the show first ran the writers had to address the challenges of week to week forgetfulness, people who came to the show without the benefit of having seen the first five seasons etc.
These issues bug me as much as anyone, and they make for great discussion. Re binge watching the show this past month has me bristling and cringing over lots of this kind of stuff. I'm trying to handle it gracefully, but I was shouting at the screen about this very subject just a few days ago.
I'm a Spuffy, so personally...I would have taken Spike up on his offer, but Buffy couldn't and shouldn't have. Would have ended up like some twisted form of prostitution given their relationship, which was already twisted enough.