Even if I find a card with a good BIN, will it not work on all merchant sites?

[Mironionovlol]

There are many variables at play. Each merchant has it's own security measures which looks at things like geolocation of ip-billing, email, bin (may be on blocked list) etc. etc.
Best thing to do is just try it out. I remember a friend found a random amex bin (travellers cheque) that would work for 5k+ on gucci and other designer sites, but other bins with high balance wouldn't hit. All depends on the site/merchant and their own individual fraud system.