I have mixed feelings on upfront fees, but I don't consider them a bad sign per se. It really depends on other factors, not the fee itself; for example:
1) Many dropshippers and wholesalers who charge a registration fee refund it as a credit towards your first order. The purpose of this is pretty simple -- it weeds out time wasters and dreamers and saves them endless hours of dealing with people who will never, ever place an order.
2) Different sites have different levels of access for non-members. If you can't see any prices, availability, order terms, freight costs etc. without paying a fee, generally I'd say try to contact the company to get this info. If you can't get serious and concrete info about the availability, prices and shipping of anything in the store without paying a fee, I wouldn't bother with them as a general rule of thumb (unless you can find significant positive feedback about the company online.)
3) Don't be afraid to ask what the fee is for, especially if you don't get it back (if you get it back, it's kind of obvious what it's for.) Some company that charges hundreds of dollars for registration, doesn't offer it back, but has an affiliate program that pays based on sales of memberships, should be avoided like the plague - they're clearly building their business model on the membership sales, not the product sales. There's nothing wrong with selling memberships, but experience and the sordid history of e-commerce all but dictate that most companies can't make a business out of both ends. Their focus should be on being a supplier, not on being a community (by contrast, if Salehoo started suddenly selling pallets of sunglasses or whatever I would just laugh it off, since their strength and indeed their entire business model is based around being a community, not a supplier.)