Yeah the really good deals on TDB are hard to get commitments on without Discord, but I've still managed to send them ~$23k in merchandise in the past 6 months without access to that. If this is new to you I'd recommend BFMR to get a feel for things, they are fantastic and simple in their website and process, but don't offer the best prices or the most deals. TDB is also great and EMB has become a favorite of mine as well. I like to play the field and work with 4-5 buyers groups. They all buy slightly different items at times, offer differing prices, and quantities that they will buy. This way I can pick and choose based on who has the better price or who is still buying more of the item, etc.

What card you use depends on whether you're trying to meet a signup bonus or what item you're purchasing from where. If you're not trying to meet a MSR then use whatever card will earn you the most rewards from that merchant. Also helps to have a 2% earning non-AMEX card if you plan to buy from Costco as they will not take AMEX. Blue Business Plus is a good choice for merchants that don't fit into any categories that you'd earn more than 1-1.5% on with other cards. Business Gold I wouldn't use unless to meet spending or a good AMEX offer. Business Platinum I'd use for MSR or AMEX offer/Credits. Use shopping portals to maximize rewards on purchases.