I confess, I’ve been there and done that and got the teeshirt, but I always try to be fair and honest about it. Anything I’m not interested in, or that looks crappy, I’ll just give it a pass (did that recently, a thanks but no thanks). Times when I lucked out (like the WK30), I tried like Hell to find some fault with it just to appear “unbiased”. 
Ages ago, one seller did the buy-then-paypal thing, and I gave a thorough review, then they offered another light for review but only wanted a quick “wow, great light!” and weren’t really interested in a “professional review” (their words). Hmf. Didn’t do that again…
To me, it’s easy to give a good review for something that’s a genuinely good product. But then again, sometimes you can’t. I kinda gave a solid “meh…” to the Wuben E01. Great value for the price (7bux!!), but I’m not thrilled with the mooshy switch, and hate hate hate SOS that keeps hitting me when I don’t want it. But for 7bux? I’ll live with it. Well, at least the only one that I actually opened. The others will probably get sold or gifted out.
But any seller that wants to “withhold” paypal bux unless/until you give a good review, is just plain stoopit. You can then return the item (zing #1) and then slam the item in a review (zing #2) as the reason you’re returning it. They gotta know that, no? So you really can’t have your bux be “held hostage” in exchange for a glowing review.
Basically, just do whatever you want, as long as you can still sleep at night. If someone you know personally went and bought you a light, would you review it, good and bad? So does it really make a diff if it’s a seller buying it for you? What’s the worst that can happen if you give it some negatives? No more reviews? Nothing lost, nothing out of your pocket. But if a fair review gives people the pluses and minuses of a product, it ain’t all that bad, even if the product is a freebie. Moreso if it’s a product you wouldn’t buy on your own (“yeesh, how many ‘tactical lights’ do I need?!?”).
Hell, I’d rather go to a trade show and get a free shirt, water-bottle, tote-bag, whatever, with some company’s logo slathered across it, than pay big bux to have “HILFIGER” or “AERO” spattered across it instead. It’s free advertising for them, so let them pay me to be a walking billboard for their brand.