To summarize the below there is not enough information yet (an why).
I am not sure that eBay
assigns the same bidder 'shorthand' to every listing.
More than one account can be assigned the same "anonymizing ID" such as A***j(1234). The such ID is not consistent for one seller on all lots bid upon. (For a while when such anonymizing ID started, it was.)
To confirm if the same "account" you need to consider the combination of the ID, A***j and feed back number (1234) to narrow it down. Even with a match, it is not 100% certain they represent just one person. Also remember the feed back number changes in real time where ever displayed.
Yes, folks can spend money. What is a lot of bidding or buying? That is a matter of opinion. I am a regular buyer on eBay
. I may not make a purchase for days or as I did yesterday purchased close to 200 individual items. (I finally spent much of my time going through sellers listing looking at many, many thousands of items, all relating to my topical collection -- the joys of a topic which does not always list in a list title.) I may make such a similar search in 3, 6 or 12 months or not for even longer. This was only the second in over three years.
That said, CK would be a fine seller for a new to the hobby collector to build a collection. Bidding does not mean winning. In the thread about SAN changing its policies, it was revcollector (if memory serves) how said while bidding through SAN he won $60,000 in bids but also placed $200,000 in losing underbids. If you say that is three losses for every win, that is not unusual. So folks, even me included, may not place winning bids that frequently and have more underbids as well. That is not shilling.
So back to your premise, one piece of important information would be what that buyer is bidding upon, win or lose. If only buying #1s or various Special Delivery or other mix of collection building material you likely go a new collector with $$$.
Now if the wins are only on 1910-1940 US but the losing bids are scatter around with no set pattern then you could just have someone who is focused on 1910-1940 but is taking a flyer bidding on other material in hopes of winning at a low bid level. And yes all losing bids are at a "low bid level."
Now if you look at CK Stamps recent (approx 90 days) sales history, there are 18,000+ items. Of those, 1200 have sold for more than $100 (that's over $120,000 in volume) with top 50 sales being $500 to over $1000. At the other end, the lowest 1000 sales were on item $2.26 or less with the lowest 60 ranging from $0.01 up to $0.26.
With the gross volume there seems no need for shilling and from all of the low selling items there is no shilling at all evident as many went a super-duper bargain prices. Your suspected bidder seems to just be doing his own thing.
Lastly let me describe what I see as the general bidding level:
1. I was shocked I lost. Items of my main focus and want.
2. I bid and stick to it until I win, except in cases of #1. Items of my main focus and want.
3. Bid to upgrade, but okay if I don't in my focus area.
4. Bid to get items in my general interest areas.
5. I haven't seen that before, let me try for it in an area of no active interest. I have no real idea of value.
6. That looks fun. I have no idea of value.
7. If I win, I know where to flip it. Also called bottom feeding.
8. Oops, did I really bid on that?
9. I bid to support the market without a concern of winning or not winning. In stock terms that would be similar to dollar cost averaging.
With the exception of #2 (this is where I am never the under-bidder) all 7 other cases will produce under-bids and a lot of under-bids. Am I schilling? NO, I am losing.
It is in the interest of most bidders to never tip ones hand about the level of the bid you are willing to make until as late as possible. So your example of the bidder bidding late in the auction is not unusual, it is wise. Nor on eBay
will all late bids show. Lot is at $10, I try to bid $20, but while doing that someone else bids and the price jumped to $21, my $20 bid will not be accepted nor displayed.
Two examples of obvious shilling, bidder climbs bid up one level at a time, exceeds your bid and then retract that bid or climbs level by level to a few cents under your bid, say $40 with you bid high bid now $40.09. You either hit the sweet spot with you bid or you just got played.
Lastly, think about this. I see an item I don't really need but it is smack in the middle of my main focus. I bid, if I win, oh well I accept it--I have trade material. BUT if I lose which is my hope, I am supporting the market (not the seller). Still not shilling. Or if I run up your first bid, perhaps you will not have money left to bid high on the other items I want. But now we are getting in the multi Nobel Prize in Economics winning area of game theory.