It's a nice suit for a mannequin, but not quite as rare as a lot of others. I'll try to explain the rarities/values the best I can... but some people know far more than me and will have to correct it likely. The oldest I believe are the double exceptional items (not this type however.) They would say A Spear of Exceptional Quality with an Exceptional under the item name. I think that those were made for so long however, that they are not the rarest.... only the oldest. I could be wrong about that.
The next to come were like this: "A Spear [Exceptional]." This could also be "A Spear Made by 'someone' [Exceptional]. These I believe to have came after the double exceptional items.... but lasted for a much, much shorter time. Due to the changing player base and the time until it the crafting system changed, these are much more rarer.
The ones I'm not sure about were Sewn by.... like "A fancy shirt sewn by 'someone'." I believe this to be from approximately the same time as the [Exceptional] items, but maybe a bit earlier? Perhaps they are much, much older though. (Could have been the first items.)
The order of value follows the rarity... and any could be increased in value if they were (blessed) a long time ago so that they can be double blessed now w/ item/clothing deeds.
Value:
1. A fancy shirt sewn by The Best Tailor (Blessed) ?????
2. A fancy shirt made by The Best Tailor [Exceptional] (Blessed) ?????
3. A fancy shirt [Exceptional] (Blessed) ?????
4. A fancy shirt of exceptional quality (Blessed) w/ Exceptional under that line 120-175m
5. A fancy shirt sewn by The Best Tailor 30-150m
6. A fancy shirt made by The Best Tailor [Exceptional] 30-150m
7. A fancy shirt of exceptional quality w/ Exceptional under that line 3-20m
8. Exceptional fancy shirt w/ Exceptional under that line See Below for Yours, not much for 1 piece
I don't know if number 1 exists for certain... and even if it did... numbers 1 and 2 might be interchangeable in value. Number 2 would even be more sought after by some of us... someone on an island south of Britain has a very, very great collection of boots in the style of number 2 on Atlantic shard... probably the rarest boots in the game. All of these could be double blessed with a clothing bless/item bless deed too I believe.
The ones in your collection would go to the 8th most rarest because it was made the most recently. Hardly anyone has any of the numbered 1 through 4 items though.... so don't worry yours is still worth something. It's a nicer set because it doesn't have a name. It would have to have The Best name to be above a no-named one.... and few have a name like that. I would value your set at between 5m and 15m though.... but I don't know where at in there. That's just because it's horned / complete/ no-named / and could fit a theme, like you said.

Good luck with the sale friend, I would either put it on a vendor and try your luck... or set it on a nice mannequin somewhere in your house for fun.