Just calling it surplus does not make it so.
A good example of a surplus outlet is the Building #19 stores in the Boston area. It really is surplus, stuff may be seconds or manufacture defects, but they will be marked if known. Typical of that type product is textiles. Will say so, right in the company ads.
Most things like siding will never be available as seconds, the manufacturers destroy and recycle the materials in factory. They don't want it available in the open market. Their good name is at risk.
All seconds are marked by some means, in most states it is the law. A stain or stamp is common on like the back or some out of the way place. Stuff manufactured on an assembly line that does not pass first inspection can not be sold as first quality goods. They will have a repair area in the factory to correct many of the items. Those must be sold under a label of something like Remanufactured or similar terms. Common with power tools.
Most real surplus has a source. Much of it is something like a product over run or end of run products, first quality, no buyer, the manufacture dumps it on the surplus market just to get rid of it. Lots is some type of insurance claim, warehouse had a fire, water leak in roof, etc. Insurance company wound up owning the product after they paid off the claim. They in turn dump the products on the surplus market.
You can get things like automated warehouses. Fascinating places if you have ever been in one. Huge acres of tall bins with robots running around. Each item is in bins with bar codes. No humans doing much of anything. The computers stock and pull goods. When a particular bin gets very low on product that is not a good use of the space. At some point the computer will purge the bins with few items, if not being restocked. That all gets dumped into pallets and sold on the surplus market. Bits and pieces of not too much of anything at super knock off price. Was how Bldg #19, got a lot of stuff.
Lots of the products where the packaging got messed up in the warehouse handling or it was damaged or is a clean out type process to purge a storage area. The volumes are not typically huge.
In my local area the surplus stores are sort of one of a kind. They get it from lots of sources, you can go there and sell them just about anything, as well as buy. Lots of their products are rip outs from some contractor doing either a house over or factory or whatever. You must be able to judge horseflesh. Usually the store managers know the history and where it came from. Lots of ours is bankrupt other stores or lumber yards or hardware stores or whatever. A way to dispose of quickly and cheaply the left over bits and pieces of inventory. Goods will have the prior store tages and prices with some sign saying final price is some percentage of that. You can wheel and deal for lots of stuff, especially if buying all in stock at the time. Only way to do business.
I got 800 square feet of a super nice vinyl solid siding, I've never seen used anywhere before. Probably industrial product for $30. Same with windows, can get first rate vinyl replacements for squat, problem being getting enough matching ones in the right size to do an entire house. Some surplus stores will have enough, they are the outlet for a particular manufacturer to dump product they have no immediate sales for, or returns or cancelled contracts or whatever.
I would be wary of a chain store operating in a lot of states. Especially if it doesn't have a sort of flea market type atmosphere. Just about all real surplus stores do.
Lots of those stores might be junk shops that import super low quality stuff targeted at a particular type buyer. Junk is junk, you have to be able to recognize it. Fair number of those type stores about. Sort of a very low grade K-Mart's selling even more questionable goods. Their supplier chains are just like any other big box store, manfactured to their specs.