Bellow is the complete history of all of the recent auctions from this seller. Looking at the history, I'd have to assume that you were the bidder with (673) feedback, since you bid on four items and lost out on three of them.
The bidder with (3691) bid on all of the other auctions, except for the auction that you won, but judging from the sale prices, I'd be inclined to think that this bidder was not a shill bidder, but someone that won the auctions, but had to back out for some reason. I only say this because they bid a single time in all cases and won the auctions. They didn't even outbid you, by much, from your previous bid to beat out the other bidders. The seller also won an auction that no one else bid on, which makes no sense for a shill account to do, unless the shill bidder is really good at obscuring their actions. I really doubt that would be the case.
That's also a lot of feedback for a shill account, and looking at that eBay account's history over the last 30 days, only 37% was with this seller, the rest was on similar items from other sellers. That leads me to believe that you and the seller were just unlucky because the original winning bidder backed out, or wasn't able to make the purchase for any number of reasons. This account could be a reseller's account or someone from another country that wasn't acceptable to the seller. It's hard to say without more probing and details. It's difficult to do this anymore, because as
Foul mentioned, eBay obscures a lot of the details, making transparency very difficult.
In this case, I've often counteroffered to the seller by offering slightly above what the bidder previous to you bid. The seller will often negotiate, realizing that at least they have sold the item to you for more than the previous bidder would have offered.
Maybe someone from The Acaeum was the winning bidder originally and could explain what happened.
AD&D Dungeons & Dragons Tomb of the Lizard King Module I2 SEALED SHRINK
0***4Feedback Score (3691) C $372.00 2 Dec 2021 at 6:28:40pm PST
o***rFeedback Score (673) C $367.00 2 Dec 2021 at 6:28:58pm PST
o***rFeedback Score (673) C $300.00 2 Dec 2021 at 6:28:32pm PST
2***3Feedback Score (2504) C $100.00 25 Nov 2021 at 6:37:21pm PST
Starting price C $99.99 25 Nov 2021 at 6:29:00pm PST
AD&D Dungeons Dragons Against the Cult of Reptile God Adventure Module N1 SEALED
1***cFeedback Score (3691) C $341.00 2 Dec 2021 at 6:43:40pm PST
p***8Feedback Score (673) C $336.00 2 Dec 2021 at 6:43:58pm PST
p***8Feedback Score (673) C $306.00 2 Dec 2021 at 6:43:56pm PST
k***oFeedback Score (10) C $235.11 2 Dec 2021 at 6:43:51pm PST
1***1Feedback Score (1618) C $100.98 26 Nov 2021 at 5:52:01am PST
Starting price C $99.99 25 Nov 2021 at 6:43:59pm PST
D&D Dungeons and Dragons Master of the Desert Nomads Module X4 SEALED SHRINK
_***0Feedback Score (3691) C $302.00 2 Dec 2021 at 6:24:58pm PST
4***3Feedback Score (673) C $297.00 2 Dec 2021 at 6:25:23pm PST
i***iFeedback Score (10) C $205.01 2 Dec 2021 at 6:25:22pm PST
Starting price C $149.99 25 Nov 2021 at 6:25:27pm PST
AD&D Dungeons and Dragons Danger at Dunwater Adventure Module U2 SEALED SHRINK
7***1Feedback Score (3691) C $211.06 2 Dec 2021 at 6:37:10pm PST
k***7Feedback Score (1618) C $208.56 2 Dec 2021 at 6:37:26pm PST
k***7Feedback Score (1618) C $208.56 2 Dec 2021 at 6:37:25pm PST
k***7Feedback Score (1618) C $100.98 26 Nov 2021 at 5:52:23am PST
Starting price C $99.99 25 Nov 2021 at 6:37:28pm PST
AD&D Dungeons & Dragons Oasis of the White Palm Module I4 SEALED SHRINK
4***0Feedback Score (3691) C $199.99 2 Dec 2021 at 6:48:44pm PST
Starting price C $199.99 25 Nov 2021 at 6:49:06pm PST
AD&D Dungeons and Dragons Dungeonland Adventure Module
EX1 SEALED SHRINK
u***4Feedback Score (3691) C $190.50 3 Dec 2021 at 4:23:44pm PST
i***rFeedback Score (836) C $188.00 3 Dec 2021 at 4:55:38pm PST
i***rFeedback Score (836) C $188.00 3 Dec 2021 at 4:55:38pm PST
i***rFeedback Score (836) C $188.00 3 Dec 2021 at 4:55:37pm PST
Starting price C $99.99 26 Nov 2021 at 4:55:45pm PST
D&D Dungeons & Dragons Blizzard Pass Solo Module M1 with Pen SEALED SHRINK
r***5Feedback Score (673) C $119.39 2 Dec 2021 at 6:18:11pm PST
4***4Feedback Score (943) C $116.89 2 Dec 2021 at 3:25:48pm PST
_***4Feedback Score (59) C $100.00 1 Dec 2021 at 2:35:54pm PST
Starting price C $99.99 25 Nov 2021 at 6:18:15pm PST