Are you seeking advice on websites which offer stamps or good quality dealers? In my opinion there is a big difference between these two things. Our hobby is flush with websites and venues offering stamps for sale but it is much more difficult to find good dealers who can support you as a hobbyist.
In my opinion a good dealer is one that who will be willing to assist you learn the hobby and avoid costly pitfalls. They will consider you a long-term investment, seek to build a relationship, and not just consider you a quick transaction. Unfortunately finding this kind of dealer can be difficult on any level never mind finding one who successfully has transitioned to this kind of relationship to an online website.
Having recourse like mentioned above with the APS site after a purchase does little good if you do not know what to buy or what to look for and buying randomly on auction sites like eBay
, Hip, and Delcampe can be the equivalent of a crap shoot for less experienced hobbyists. Yes, there are good some good sellers on these sites but the sites are mostly targeted at facilitating transactions and not designed for education, support, and developing a beneficial relationship.
You have already come into one possible solution; this community. Folks like 'KenR2' (Ken Relyea) and 'paperhistory' (Matthew Liebson) are eBay
sellers who use communities like this one to assist hobbyists, offer education, and provide additional insight. There are others so it pays to review this community's threads and note who is willing to help in ways beyond simply making online listings. You can also search to see if a dealer has a website, gauge its educational value, and note how responsive they are in replying to inquiries.
Avoid buying by 'sorting by lowest' on auction sites, avoid putting much stock in online auction feedback scores. These are things that scammers and care-less (as in 'I could care less about hobbyists') sellers use to their advantage; they fully know that selling from descriptions and images tilts the playing field in their favor. Manipulating these things online is quite easy and this obfuscates differentiating the good and bad sellers.
From my chair there is much more in building a good collection and enjoying our hobby than buying cheap material from the lowest listings that a search or filter can return. Once a collector moves beyond penny stamps the journey become more reliant upon education and experience; these things can be facilitated by relationships with quality dealers.