Someone handles accounting to record management fees, pay vendors, and pay the client... 

Or what type of vendor to call in to fix it?

They are the ones without adequate insurance, commingling funds, performing work without a contracts, relying on insured contractor’s /vendors etc. 

Granted I run a larger operation but I negotiate pricing annually with all vendors. 

Perform the same task in each app to ensure a fair comparison.Load a property with pictures and details.Market that property.See what your marketing looks like from the public's perspective.Submit a fake application to see how easy the process is.Run a credit/screening report on yourself.Enter a maintenance request, assign a vendor, and attach a fake invoice.Enter charges to the tenant's ledger.Enter recurring charges and automatic late fees.Sign documents electronically.Run owner reports.After testing a few apps, one should clearly stand out.

Could the Seller just pay some of the vendors through the net proceeds?

Unfortunately, this is the primary reason I do not use third party vendors for rent payments, and I have posted on here a few times about this (usually when someone posts about a fraudulent clawback), despite the throng of enthusiasm for the continued use of these methods.Each one of my tenants is given a pre-designated bank account (idenitifed in the lease) to which they deposit rent. 

We're basically vertically integrated construction vendors.

I can pre-authorize an amount and if the expense is below that, the vendor can do the repairs without talking to me live4) Urgent maintenance issues - say a pipe breaks, or tenant is locked out.

@Calvin HaywardReal estate agents frequently provide closing presents to purchasers and vendors as a signal of gratitude and celebration of a thriving deal.