Welcome to the forum!
Although Main Merch is not in our directory, they should be safe to deal with.
I've checked that their site has been online for 5 years (it is safest to deal with suppliers who have been online for at least two years). Also, the address on record with their hosting company matches with their website. They accept checks and major credit cards as payment methods. We suggest using a credit card though as they give you a comeback if anything goes wrong. For dropshipping with them, you will be invoiced at the end of each month for all shipped orders.
Do note though that we cannot guarantee the safety of any suppliers outside SaleHoo.
Hope this information helps.