Hi Tandoorii,
Shellfish/seafood allergies are caused by muscle proteins in these foods, such as parvalbumins in fishes and tropomyosin in crustaceans. Being allergic to these does not mean one will be reactive to chitin, a chemical found in the shells of many crustaceans. It is a complex carbohydrate known as a polysaccharide, not a protein.
You are correct that unregulated supplements such as chondroitin and glucosamine come with no real guarantees as to composition, potency or purity. You pretty much need to rely on the integrity of the manufacturer and as such might be best served by purchasing products made/marketed by well-known national or international companies than off-brand ones from less than fully developed countries.
And you make a good point about chondroitin and glucosamine marketed as bovine-based actaully containing shark fin. Bovine cartilage obviously is the less costly of the two.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual and should not be construed as such.
Regards,
DocVikingo