It's always a good idea to check out resources to ensure their reliability. Look for reviews from other customers who have purchased used gear from the brand. Another good resource is to check out the brand's social media accounts and see how they interact with their customers. Are they responsive to questions and concerns? Do they provide helpful information and support? You can also check their digital channels for infos about the people behind these boutique brands. You can use that info to run some personal checks, like any history of filed complaints against them, etc.
By doing your due diligence and checking out these several digital resources, you can get a better sense of a brand's reputation and whether they're likely to provide the level of support and aftercare you need for your used gear. Good luck with your search!