Sora is below Tiagra in quality...and has thumb shifters rather than a small black 'return' lever behind the main brake lever (which many folks dislike). You can search the threads and read quite a bit about that difference. Personally, I didn't think the Sora's were that bad (on my first bike). But any upgrade path would be a bit trickier if those are 8-speed (Tiagra would be 9-speed and tricky enough as is). I'd certainly double-check and possibly use that to haggle the price a bit - but it wouldn't necessarily keep me from buying the bike if it fit.
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur