An armorer will pay you slightly more for useless breastplate than a common merchant who offers typical items.
You can trade foreign currency in Vivaldi bank on the marketplace in the free city of Novigrad. Luckily, there is an exchange office in the game. It makes other two useless in standard transactions with merchants. The latter is a commonly used currency in the game world.
If you want to repair your equipment on a regular basis, buy better armor and swords, you must utilize all ways of increasing your income. At least, ones linked to monster slayer profession. Expenses are large and there are only few sources of income. At the beginning of the game, Geralt has trouble with accumulating decent amount of money. In the original books, witchers never had much money. Summing it all up, if Willis has 15000 and you do the procedure 20 times you can make 300 000 in under 5 minutes.Witcher contracts are one of better ways to earn money quickly Do this a few times and you can loot all the money from all the multiple Willis NPC's on the floor.Meditate for 1 hour and he will respawn and die again (the other body doesn't disappear neither does the loot).When he dies he falls to the ground and drops everything he owns (his money too).Once close make sure you put the beehive close to Willis so that the bees kill the NPC.When you're close to the village make sure you bring the beehive through the back (from Willis back) so that you Aard isn't blocked. Knock the beehive down with Aard and continue taking it through to Willis.If there isn't one meditate for 1/2 hours. After giving him all your money go next to the hut by the river close to Willis and there should be a beehive in the trees.Buy whatever you can from him (the more money Willis has, the better).First go talk to Willis in White Orchard (You must have done his quest to unlock his shop).Since the video can be brought down I will explain what it consists of:
In the future this bug may be patched out I didn't do it myself because I wanted more of a fend for myself experience but you can do whatever you want so I'll put it here for the sake of spreading knowledge. You can perform the bee-hive glitch with Willis in White Orchard to get yourself a huge amount of money in a really low amount of time.
P.S: There is also a "naughty" way to make money found after patch 1.2 (Blood & Wine) So if you wish to explorer a bit feel free to get the loot off these houses and put it to good use. So I found myself getting almost 3000 just from loot of 3/4 houses.
These houses are filled with jewellery like necklaces, rings, etc (Some even have diamonds lying about). Spread throughout Toussaint there are open Houses (normal houses where people live in).