I would not buy somebody else's bucket unless I could either personally examine it in detail or have someone I trust do it. The quality of workmanship on these things runs the full range from show quality (which you'll typically pay a premium for) to "OMG! How could that thing possibly function?!?!?" If you are looking for an "affordable" bucket, you're more likely to have to deal with quality of workmanship issues. I would be more comfortable buying a used "kit" car than a home-fabricated one.
I haven't priced used buckets, but I think you are probably right that you will pay less for a used bucket than the comparable kit. Personally, I really enjoyed putting my Spirit car together. I was able to customize it for my tastes and I know the quality of the workmanship as I touched every single fre-fabricated part and did most of the additional fabrication to make my bucket unique (George Barnes (GAB) did the critical fab work on the Jag conversion, but then George is a fab genious).
Also, don't let the term "kit" fool you. Although you can get a very complete kit from Spirit (I don't know how complete the other vendor's kits are), you will still end up having to source A LOT of parts. This can get very aggravating at times. If you don't have the patience to fit and re-fit, chase parts, order parts on the 'net and have to send them back because they weren't what you expected, etc., then a used bucket is probably the way to go (with the above caveat).
Good luck on your project!