I'm not sure what makes the seller think it's a Cinelli. Those dropouts, lugs and BB shells are on tons of bikes.
I didn't see any other signs that it was a Cinelli, and having 105 would mean the BB is most likely English.
Could be a Cinelli frame, but I don't see anything Cinelli in the photos except a widely-sold BB.

Let's summarize a bit, and I'll relay what I know and believe, right or wrong. I'm not a Cinelli expert, or an Equipe expert.

1-The "OEM" Equipe was designed and produced under the supervision of Andreas Cinelli, years after Cinelli was sold to the Columbo family. It was produced in a very limited (75 or maybe 150) quantity under an agreement between Centurion and Cinelli, in a shop an hour from Milan, possibly called the Chichiro shop. The same shop also made at least some SC's and was a limited production, higher end shop. The Equipe was allowed to have two small Cinelli decals, on the DT above the shifters, and on the L chain stay. It had an externally lugged fork crown, chromed, and chromed head lugs that did not carry the tiny flying C logo. It had chromed Columbus dropouts and fork ends, a polished chrome R chain stay, and a chromed frame under the paint. The flying C was panto'd on the seat stay caps and the brake bridge, along with the fork crown. There as no serial number, and the widely-sold Cinelli BB had only the frame size stamped on it. It had what can kindly be described as "incongruous" decals by Centurion on the head tube and DT. I've seen them in lavender/black and orange/green. For a long time, I thought the orange/green were repro's, but today, I'm convinced they were OEM.

Original equipment were Fiamme Hard Silver tubulars laced to Miche hubs. Freewheel was Regina, RD, FD, and shifters were NR. Crankset was an Ofmega Mistral on an Ofmega BB. Ofmega also produced the headset and the pedals, called Sintesi's, also marketed by Avocet, I believe, with a composite pedal body. Seatpost was Gipiemme, with a Cinelli Volare suede saddle, Cinelli bars/stem, of course. Brakeset was Universal AER, some of the prettiest (and beefiest) calipers of the era.

2-There were a couple of prototypes made. The best source indicates they were matte black with red decals. I have seen one, owned by a BF member. It was definitely matte black. It definitely had red decals, saying Centurion on the down tube. They were as ugly as those on the "OEM" Equipes. I do not recall if there was any chrome whatsoever on the prototype I saw, and I don't know the component configuration. The owner of that liked it enough to be interested in the CL sale of my 56cm Equipe, as he was the "other buyer," which, at the time, I wasn't sure existed (CL negotiations being what they are, sometimes). It was only after I bought that Equipe, much later, did I encounter Tom, and trade photos.

3-The "blue group." These seem to exist without OEM decals of any kind, neither Cinelli nor Centurion. They do have some or all of the panto stamps as mentioned above. They do not have the chrome lugs of the Equipe. One I've seen has the same externally lugged fork crown as the OEM models, and others have the internally lugged chrome fork crown commonly found on the Cinelli SC's. A couple I've seen had NR FD/RD/Shifters, and one, at least, had the Ofmega crankset/bb. I've never seen one with anything close to OEM Equipe decals. It's my belief that they were likely made around the same time, around the same place, and perhaps by the same crew, as the Equipes, perhaps with the same jigs. The most likely reason they are not decaled is because they were not part of the agreement, and likely would not have had approval to have Cinelli decals on them. It definitely appears no one sent any Centurion decals to be applied to them, either. Whether they were made before or after the Equipe's production run, who knows? Do I think the hand of Andreas Cinelli was in them? Yes. Do I think they are Equipes? Not officially. Do I think they're the same bikes, in general? Yes. Do I think they came from the same people making SC's? Yes, at least some of the SC's.

4-The "grey group." These may be similar to the blue group. Some share the same "SC" fork, others do not. None have the chrome lugs. Most have the pantos, as the blue group does. They appear to be painted the same color as the Equipe. I've never seen one with the OEM decals, neither the Cinelli nor the Centurion. I have seen them with repro decals, such as the one on eBay. The lack of decals, it appears, would be for the same reason as the blue group. The finish on the grey group seems of less quality than the blue group, and I think that's likely due to the color of paint. Back then, the color of the paint made a bigger difference, sometimes, in paint quality. It's my belief that they were likely made around the same time, around the same place, and perhaps by the same crew, as the Equipes, perhaps with the same jigs. Whether they were made before or after the Equipe's production run, who knows? Do I think the hand of Andreas Cinelli was in them? Maybe, I don't know. They seem different. Do I think they are Equipes? No. Do I think they're the same bikes, in general? No, but they have as much chance as anything, and could be a higher level, or a lower level. I simply don't know. Do I think they came from the same people making SC's? No, but that's just what I think, and that has no basis in fact.

I've said this before: Every time I think I have a handle on this topic, new information pops up. I can only relate to the Equipe when it comes to this topic. Despite owning a Super Corsa, I know zilch about them. The man that negotiated the project for Centurion also sought out Cino Cinelli to help on the Ironman, but Centurion went a different direction, it appears on that. Sales of the Equipe were not good. The price was high, and the dealers had been selling the Turbo, Pro Tour, etc as "equals" to the best frames coming out of Italy, so perhaps the introduction of an Italian-made bike into the mix created some confusion among those trying to sell Centurions.

You know, yours could be a frame built for teams, as could the others. I never thought about that.