This idea of not having paypal integration enabled for all users is not the way to grow the platform or the buyers. You have people who are hesitant to buy huge dollar memberships to have access to the platform, and they are turned off even more by the practice of having to send an invoice after the fact instead of the buyer completing the payment THEN, when they are in the process. Even crappy sales platforms offer this integration free, and I believe it is the wrong method to try to gain memberships. I know I stopped mid setup with a store and deleted it last night because I am not chasing payments. I was one of the first high volume sellers on ebay of vintage motorcycle parts, and I was also one of the first to require immediate payment.
This was one reason why ebay wanted all instant payments and paid immediately so buyers were paying while they were motivated, not the next day or week by check or money order. There used to be so many unpaid items on ebay with money orders and Western Union pay by by check crap. I had nearly 1/3 of my early ebay sales unpaid, never rec'd any type of check or money order after wasting all my time and effort.
I see the reviews online are not encouraging either. Disabling user accounts for odd or no reasons, setup problems, no sales after a year, but steady sales on ebay of the same items from the same sellers at higher prices.
Memberships are fine, but the platform needs to give users the opportunity to see if the systems works for them before they charge a fee for a paypal invoice.