Most couches are made of pine; either recycled pallets, Wet of Saw (un-dried and un-graded timber), or KILN dried (a process which reduces the moisture of the wood); the latter being the correct product to be used when manufacturing couches.
Many cheap products are made using recycled pallet wood in most part of the couch. There are two major concerns here.
1) The thickness of the wood and therefore its strength is ; 19mm instead of the recommended 25mm.
2) Certain pallet used are in a terrible state; the wood exposed to extensive sun, rain and lost most of its strength. The use of un-dried wood (Wet of Saw), very common in the industry, is a serious concern as well. During its natural drying process, after the product was manufactured, the wood will be releasing its moisture, and mold will start forming on the fabric. It is obviously unhygienic but can also seriously damage the fabric, especially when using leather-like products such as PU or PVC.
The wood will also contract during this process, putting pressure on the joints of the frame, weakening it, resulting in a flimsy frame and squeaky noises eventually.
But more than the wood itself, the workmanship when manufacturing a couch frame is essential and participates largely in the lifespan of the final product. Glue should be used on all joints to ensure that the frame stays steady. This process is time consuming and is costly, and therefore not implemented in many factories. How can you know that the frame of the couch you are buying was properly manufactured and with the correct materials? You can press your fingers into the frame to try get an estimation of the thickness. Do not hesitate to drop your self on the seat; that is probably what will happen at home anyway... so go for it :)
If you buying online, only buy from a reputable seller with genuine good reviews. Check their T&Cs and in particular the part which deals with the warranty/guarantee of the product. Lastly, I would recommend you to be an informed and realistic buyer. I know and understand that everyone is looking out for a bargain. But after shopping around, you should have a fair idea of the “right” price point for the type of couch you are looking for. If it sounds too cheap, the chance might be that poor quality material were used.