If somebody acquired an extra suit (or pieces of) armor, would they be able to resell it as armor or just as scrap? If the former, what percentage of the armor's initial value would they get in return?
Related question: If armor were made to fit one individual's specifications, how hard would it be to resize it?
Hey there! Your question covers roughly one thousand years, apparently the entire world, and every phase and form of armor that existed in said time and place - there are going to be a lot of different answers to this question!
Questions tend to do better when they're more specific, since they do better at catching the eye of the relevant experts; you might want to think of a more specific time, place, and kind of armor if this question doesn't get much attention.
I will answer your questions as far as possible, focusing on the 14th, 15th and 16th centuries, the period of plate armour, and the main area of my reading.
For the first question, would people be able to resell armour -as armour-? Yes. There is a variety of evidence for this, some circumstantial and some less so. Firstly, we know that armour was used over decades and even centuries, most particularly maille. In the Glasgow museum there is a 14th century maille shirt with 15th century alterations and repairs. Former Glasgow museum curator Tobias Capwell in 'The Real Fighting Stuff' (a survey of the museum's collection) writes that maille shirts like this could be repaired, re-used and altered for decades, just by adding and removing enough links. So a 'used' maille shirt would retain much of its utility as armour. Other armours, such as cloth armour (jacks, gambesons, etc) and brigandines (small metal plates rivetted to the inside of a canvas garment, like a sleeveless or short-sleeved doublet) are also easily re-used, and are often mentioned in wills and inventories (such as that on the death of John Fastolf in 1459) as goods of value (not valued as they would be if they were just trash).
Plate armour was also re-used and re-sold, though the degree to which this is done, and how it was modified for its second owners, is an interesting question. As related in Edge and Paddock's Arms and Armour of the Medieval Knight, in 1382 the merchant Franceso Dantini sent his agents to Genoa after hearing of the rout of a company of lancers, so that he could buy as much armour as cheaply as possible for resale on the secondary market. Dantini was not a merchant in scrap, but in armour (both maille and plate), so this cast-off or looted armour would most likely be resold as armour, not scrap. This makes economic sense. Unlike some other goods in the middle ages, the -labor- that was put into making armour accounted for the vast majority of armour's cost. According to Alan Williams in The Knight and the Blast Furnace, by the 16th century the cost of the metal was as little as 10% of the cost of finished armour. As a product with a high value added, scrapping armour when it wasn't necessary didn't make sense. That said, there is only limited pictorial evidence for the use of 'old fashioned' plate armour. It is unclear based on my reading and research how long the 'lifecycle' of a piece of plate armour was before it would be repurposed or modified beyond all recognition.
Alan Williams speculates that the 'half steel' used by Nurnberg armourers included recycled plate from armour, which would be one example of 'scrapping' armour. There is also one breastplate profiled in his book that appears to have an upper part (for the cheast) made from a late 14th century breastplate, and a plackart (defense for the upper abdomen) of a later date. They were heat treated together, hinting that this armour was cobbled together sometime in the late 15th century out of new and recycled parts.
As to what the difference in cost would be between 'new' armour and 'used' armour, I have not seen the two distinguished in my reading. Speaking of 16th century mass-produced 'munition' armour for infantry, there -is- a price difference between orders of armours direct from armouries and orders from wholesaling merchants in the Netherlands and Northern Germany (with the latter being significantly cheaper) - the Nurnberg and Augsburg guilds constantly complained of the competition. It is not clear how much of the armour the wholesalers was 'new' and how much was 'pre-owned', though. So it is hard to say how much armour depreciated in the secondary market.
Regarding how full harnesses of plate armour would be resized, I have not read anything that wasn't highly speculative and so will not comment.
Sources:
As cited in text, above.