No significant buyers remorse here. An occasional "OMG did I just spend THAT much" but that is all. Never been ripped off. Occasionally something goes astray in the post, but sellers (or Amazon/Ebay/PayPal) have always refunded me.
These days it seems more risky to be a seller rather than a buyer on Ebay.

(a) Certainly have had Deagol's experience of Spending way too much. The last 3 were a Tolkien lot from England, the cartage was almost equal to the cost of the lot and orders related Tolkien Maker' exhibit items and FoG, in all forms I could get in the US & UK.

(b) Prepaid for the HC boxed set of boxed deluxe volumes: H, LR, S, CoH. Waited. A couple of times the seller told me the release was delayed. I learned others had copies & contacted HC. End game: Seller went broke with my money, no books, no refund. Eventually got a set though.

a) I had pre-ordered the deluxe Maker of Middle-earth way back when it was first (erroneously) announced. Not pulling that order meant that I ended up getting it a little cheaper, but when I finally received it, I was ultimately disappointed. It was really nice - just not enough to justify the cost. I may have felt differently had I been able to attend the exhibit. I ended up returning it.

b) I've had plenty of poor experiences, but perhaps most memorable are the two times I've ended up with notices from the USPS about my package being destroyed/lost. The first time I received a partial package of a Tolkien lot I ordered on eBay. The reason for the order was a Nasmith-signed first edition illustrated Silmarillion with the less common dust jacket. That was one of the missing books. The second time was another eBay order, this time of an unsigned, but Fine copy of the same book. This time I just got a notice and no package. My theory is that there's a postal worker who is a Silmarillion fan who likes to keep on eye on my mail. I still don't have a copy of that dang book!

(a) Being completist means generally that i don't regret buying anything that shows up as expected. However i have (on many occassions) purchased books only to discover i already have a copy in the collection.

(b) Amazingly (considering the number of books i purchase) i have not had any drastic experiences. The worst offender (and persistant) is Amazon. Poor packing (even with expensive books) has resulted in damaged books on a regular basis and have to go through the painful effort of returning (though sometimes they send a replacement and tell me to keep the original one). I try to avoid them when it makes economic sense (i.e can get cheaper or pay a little more elsewhere).

waking this one up again

a) a first edition LotR set (9th, 8th,8th) with facsimile DJs. The books were in near fine condition and the set was not very expensive but still. What was I thinking?

b) bought a near fine set of 1990 LotR Unwin PBs with the Ted Nasmith covers on ebay. The seller refunded me the money claiming he had stopped shipping internationally for the time being. The set was relisted and sold about 10 seconds after the re-listing for twice what i had paid.

(a) The recent Folio Society Deluxe Limited LotRs set (illustrated and signed by Alan Lee). The books just don't do it for me, and for the price of the set the flimsy, easily damaged slip case is poor IMHO. Also the recent deluxe History of the Hobbit. Paper quality just wasn't good enough I felt. Should have just charged more with better quality paper.
(b) A first impression deluxe Beren and Luthien I got at a bargain price on eBay - it turned up badly damaged. Kept it as a reading copy.

This could easily turn into a moaning thread.