Can't think of any that I only saw once, cause unless another local is looking for a figure, after I see it once, I don't really pay attention anymore. Only figure (I think) I have never seen in stores was HA Mudflap. Thankfully my cousin grabbed one for me for $15.

 

Human Alliance Soundwave, Transformers Universe Unicron, TFE Bulkhead

 

Sunstreaker circa 1985.

And i never saw the original Sideswipe or Trailbreaker at retail. At all.

 

G1 Bluestreak. I only saw it once as a kid, and I ended up getting Mirage that day simply because I liked Mirage better. Also, the gift sets of the combiners I only saw once each. The rest I either saw multiple times or not at all.

In recent times, I've either seen figures more than once (RotF red Rampage is the only figure I've just seen twice) or not at all. Notably, I don't "hunt" a whole lot either, as I don't enjoy "hunting" as much as "shopping".

 

Universe Starscream. It was like I saw a ghost.

 

Funny how these things vary from country to country..... Bluestreak was a major shelfwarmer in my part of the world, often the very last Autobot car left.

 

For real, man  

 

MP Starscream. Held him in my hands before Xmas, and thought I could get him after. Oh how wrong I was.

 

Space Case...oh, and DOTM Starscream ¬_¬ You wouldn't think the latter, but man did we have fun finding him for my sister.

 

The couple I remember passing up and never seeing again were RTS Perceptor and RTS G2 Prime. Sadly I NEVER seen Junk Heap at all.  

 

G1 fort max...man i wanted that ****** bad..but to rich for me parents blood so....i bought him when i got older 

 

inferno was the only one I can remember too. I was out of the tf loop at the time, had taken a break from collecting. saw him on the shelves and thought "hmm this looks pretty cool." put it in the cart. decided walking through the store I didn't want him and put him back.

luckily another local guy helped me out last year with him for just slightly more then regular retail price. seeing ebay prices I was very thankful that he was able to offer it for a reasonable amount. toy was in great shape too  

 

ROTF Mindwipe.    

And just last month, Specialist Ratchet. Got him though.

 

Robo-vision Optimus Prime: Didn't get, tried to get it later and couldn't find it.
Wal-Mart exclusive MP Starscream: Bought on sight.
DotM Space Case: Hunted, and bought on sight.
Cyberverse Shockwave: Bought on sight.
Prime Soundwave: Had them pull it from the back, haven't seen it actually on the shelves.

Never seen:
Alternators Rumble and Ravage.

 

For the most part, a lot of my "only seen once" mirrors everyone else except for Skystalker and Space Case. I saw those both multiple times. Had I known that Skystalker would prove to be exceedingly rare, I might've bought more. Our Target didn't get Space Case until a month or so after everyone else, but we got several.
Cyberverse Shockwave (only saw once, box was so damaged they marked it down half price and I used a $5 off coupon, so I only paid a few $ for it.)
Universe Inferno (bought him, but left his casemates who I only saw once)
Dark Steel (was saving for Christmas and passed on him. Haven't seen him again)
I have never seen:
Universe/Generation Legends (Bumblebee, Cosmos, etc)
HA Mudflap
Red Rampage
Evac
Specialist Ratchet

 

FE Bulkhead. Saw him once in a major toy store, and one week later, he's gone. A few days later, I found out that he's the rare voyager, compared to FE Prime, and he's being hogged by specialty shops and priced very steeply because of rarity, high demand and low supply.

That didn't stop me from getting a good deal for one after searching patiently, thoroughly and painfully.

 

Going back some years:
- Alternators Mirage, Camshaft, Decepticharge. Bought on sight, thank goodness.
- Energon Dinobots, Ultra Magnus. Got UM, had to ebay Dinobots.
- Primus with Unicron Head. Didn't get it and ended up with standard release.
- HA Barricade, HA Mudflap
- Animated Roadbuster Magnus

I'm sure there are more.

 

Altenators Deceticharge and Swerve. Bought them on site.

 

First Edition Prime: Bumblebee. Saw it once, it was the only FE Prime I ever saw at retail. I didn't buy it because I wasn't willing to pay TRU prices for something I was sure I'd find at Walmart.

 

ROTF Skystalker and HFTD Highbrow, most recently. Bought on sight, never spotted again.