What about a listing where you have 50 available and also add Best Offer.

Then write in your description that you can offer discounts for purchases of bulk quantities?

Someone with more experiuence will come along with the right terms etc (and if this is permissible)

so fior example, list an item as $50 BIn or best offer with quantity 50.

then in your description say, the BIN is for the purchase of 1 item, however, depending on quantity purchased, a lowewr "best offer" can be negotiated.

maybe even detail the prices you will accept for best offer (as mentioned in the post above) in the description.