Foxycart Payment Question
Using Jamroom
1) Not sure how it works in the US, but in the UK you might only be liable to pay tax on any profit made, ie., the commission.
2) FoxyCart does not work that way automatically. What I'd suggest is setting up several 'vendor' quotas with different levels of commission. You could then move the vendor profiles individually to the appropriate quota every month (say) prior to payout. Agreed, this might be a bit tedious if you have lots of vendors, and if that did prove to be the case, I think a relatively simple custom module could automate the process.
hth