I am pretty sure that there is a large and active market for renaissance art. Sure, big items like Rembrandts are traded rarely (but they are occasionally auctioned) but those are the +5 vorpal holy avengers. "Lesser" art from more unknown artists are traded every day and good art trades can likely put you into contact with someone who will sell renaissance art if they do not do it themselves.
But you can't just walk into a store in any city and purchase a piece of art in the exact style of your choice. There are a finite pieces, many of which are in the hands of museums and private collectors. So you need to find someone with a piece you want and arrange a private sale, or wait for an auction. There's only a couple auction houses worldwide that handle such things, and even unremarkable pieces go for well over a million dollars. People get paid to track down pieces or particular styles of painting: art dealer is a full time job.
This is also the modern world. It gets harder when you remove instant communication and reliable (and secure) long distance travel and transport.
Selling a piece of renaissance art probably wouldn't be *that* hard. Supply and demand and what. Similarly, selling a magic item wouldn't be that tricky. But finding a particular magic item, even something that's only rare, might be much trickier.
And even if you assume that the knowledge of how the create magic item has been universally lost and no known place on the planet knows how to create them, there would still be a market for continual flame torches. Every noble would want to stock his residence with it and why shouldn't he? Or is the party wizard the only one able to cast 2nd level spells?
Keep in mind that you'd really need 2-3 castings for each room in a manor/keep. It would add up fast, even for high level PCs who can do it in a couple days. Plus, the price would depend on the number of people who can cast the spell.
I went into the number of people who can cast 2nd level spells a while back using the demographics of 3e. In a city of 10,000 people there'd be 75 who could make a magic lamp of eternal light. But, that was using 3e's demographics where 10% of the population had class levels, so it'd be very easy to half the number of people to <40.
So while it's possible to have an Eberron where that sort of thing is everyday and many people can cast 2nd or even 3rd level spells, it's equally possible to have only a handful of people in a large city with that kind of spellcasting, and not every one of them will be willing to sell their services.
There might be a handful of clerics who can do so, who are the head priests of a church. Some might be willing to cast a few spells for a noble for a donation to the church, but many are going to balk at being asked to waste their divine gift. Similarly, not every wizard is going to hire out their services. Some might be reclusive and not advertise their skills, some might be wealthy and dismiss such spellcasting as "common labour", and some might be only interested in knowledge and expanding their art. And some might be adventurers who are unlikely to want to put their great quest on hold to spend a month enchanting a dude's house. Plus, not every wizard will know the spell; it's not in Basic so, arguably, the spell is more specialized and learning it would require some work.
I used the examples of trades: mechanics, electricians, plumbers. Those are pretty common skills that are not *that* hard to learn, but the average cost of a plumber visit is in the $100/hour range. The parts aren't that pricey and most of the time the job isn't *that* difficult, but you're paying for the expertise and certainty that they're not going to eff up something that will cost 10x the price to fix. Plumbers can pretty much charge what they want.
So when there's 5 people in a major city with the expertise, time, spell knowledge, and willingness to do something like enchant lamps, then they can charge pretty much whatever they want. So the 50gp base price goes out the window. They can charge 100gp or 500gp or more. Even a wealthy nobleman is going to look at their 15-room manor and the 250gp price, and decide it'd be easier to buy 37,500 pints of oil. (If lamps in only half the rooms are lit at any given time, the noble could light their house for a decade for the same price.)
Now, a noble with more gold coins than brain cells might opt to buy a couple eternal lamps or continual torches, but they're unlikely to be everywhere.
On the other hand, a wizard's tower or cathedral to a sun deity might have braziers every dozen feet. Because they can.