Quite. Art dealers routinely go on working vacations to Europe, so it's realistic. Also, consider the genre; all parents in Gen X Teen Dramsties are away for weeks at a time on either business trips or vacation. So just go with it, and be glad Joyce didn't get flanderized as a miserable drunk who blamed Giles for every time Buffy got a bruise on her.
Do we know how much time passes between each episode? Joyce was definitely out of town in Halloween since Buffy's house was empty and I don't think The Dark Age occurred too long after that because why would Ethan hang around Sunnydale for an extended period of time doing nothing? Reptile Boy- What's My Line Part 2 might only occur over two weeks in-show rather than the six weeks due to airing schedule.
We know Halloween is 31st October and Surprise/Innocence is Buffy's birthday. Given that she's a "Capricorn on the cusp of Aquarius", Buffy's birthday is 19th January. Reptile Boy is obviously before Halloween so we're probably looking at an October timeframe for that one. Lie to Me is definitely November. Don't forget that the female vampire stole the book containing the cure in Lie to Me and Spike and Co manged to decipher it in What's My Line Part 1. Spike isn't going to be sitting on his laurels and he talks about "running out of time" which means that What's My Line Part 1 isn't too long after Lie to Me and obviously WML Part 2 carries on directly from where part 1 ends.
Halloween 1997 was a Friday. We see Buffy at School in Lie to Me which means there is at least a two day gap between Halloween and Lie to Me. I don't know whether they have a week long half term holiday in America or not, if so then that would be the week starting the Monday after Halloween (if it falls around that time) because we saw Buffy and the scoobies at school on Halloween where Snyder roped them into volunteering to chaperoning the kids around (which Spike kind of ended up doing instead).
It's possible that Lie to Me- What's My Line Part 2 all occur within the same week following Halloween.
I like your reasoning, Deadly Duo, even if I suspect that the answer is pretty much what AlphaFoxtrot writes: That the parents are out of town every time the storyline needs them to be away, with no further thoughts about it.
Buffy doesn't really have a good internal chronology, largely due to the May episodes taking place in about one day. The fall episodes, one episode is one week or so. The mid season episodes, closer to two weeks each. May Episodes, heck, 1/3 third of the seasons episodes could be compressed into two days. But you also have the Late Winter/Early Spring run where a new episodes airs every three to four weeks. I don't think the episodes have one, but you probably could force them to fit, Joyce was going on a Fall European Art Trip, which is typically going to be the post tourist season, and would be the time to go for the best deals.
Yeah, what others said - I assume Joyce was out of town on a work-related trip, perhaps looking at possible art for the gallery. Buffy was mature and old enough to be left alone, so no need to hire a sitter or someone to stay with her.